Is there an easy way. Yes home demo is a great idea if possible. I have learned that deeply and hope others can read this thread.
The funny thing is that an NDX2 and a 250 were a dream at one point and the 300DR was not even something I could consider… it’s all based on perspective.
I am I think seeing I am at the top of the speaker tree (for me) and the rest of the system needs to catch up.
The design of a really good passive crossover for multi-amplification is complex. Naim has always been in favour of active filtering, the first SBL, and no doubt DBL, were initially produced for active use; the SBL, DBL, mono-amplification filters for passive use came later.
I know friends who have B&W 802 and 804, each driven by two Nap 250s in bi-passive mode; they no longer want the amplification with a single amp. On my Linn Isobarik I have three Nap 250 Olive in passive tri-amplification it’s much better than 2 X 135; even with two Nap 250 it’s more than 135. For 25 years, according to what was explained, I thought that passive bi-amplification was less compatible, except that good passive filters with real separate cells like B&W and Linn play perfectly with Naim!
You have to be able to listen to see for yourself.
I am going to experiment - so you think I could get results from a 300DR on the HF and the 250 on the LF?
From experience the 250dr is a good amp however, it needs very careful matching to the speakers - I think it sounds a little weak driving B&W’s. However, as the guys have pointed out above - you need to improve the source path as well. I would suggest you push the boat out and get a second hand 500/552 combination - it would be a match made in heaven expensive yes - but s/h prices are lookng attractive. The 500/552 is a well known combo for the higher end B&W’s… Ultimately you will need to improve the dac/streamer but that could wait…for a while, at least until you have picked your jaw up off the floor…have a good xmas… (don’t be tempted to add subs … not until you have upgraded the rest)
As @Gazza and many others have highlighted, there are many variables in this hi-fi lark, with some ‘known challenges’ and some of those nasty ‘unknown unknowns’ or unforeseen aspects, hence why a process-like approach is advocated, so as to ensure you can address, understand and limit the influence of the variables, and introduce constants, which also helps you to evaluate changes in kit, obviously for good or bad.
In business, there something called ‘analysis paralysis’, where too many inputs/opinions (as well intentioned as they are), can start to muddy think, approach and stifle actions – I think the advices from B&W (as an example) don’t fit your cause here. Assuming you want the highest quality replay through these wonderful 'speakers, then:
1- Understand and address any room issues
2- Get a better/best source you can - better source components will enhance detailing, bass performance and whole musical picture – this is what adding a 555PS will do to an NDX. Note the words around bass – you’ll have more, with more weight to the music. Even better with say an ND555 (obviously).
3- Address the pre-amp, as this also heavily influences the replay – you need to get more through the amps per the source upgrade.
4- Now, on the basis the 300 is ‘good enough’ for the B&Ws, I’d address a power amp change last i.e. it’s just not sensible to look at this first, when you don’t know what the source upgrades will deliver in to the speakers and your room.
I stand by my words that, ultimately, these 'speakers will benefit from a far better/more powerful amp – but it’s really not step one. The real risk is that you improve the amp now and then find an improved source over-bass(es) your listening room. You don’t need Statement-level, it’s just that before the NC350s arrived, as good as a 500DR is, some 'speakers challenged it. The usual wisdom is a great amp with relatively modest speakers hooked up, is far better than a modest amp with big speakers (read as often ‘demanding’). You can tweak performance with cables, especially 'speaker cables, but I’d stay away from this at the outset. Stick with Naim’s standard stuff.
Of course, nothing above stops you from playing with bi-amping etc, on the basis this is easy to achieve.
OK so I have now spoken with Naim. Interesting feedback.
First - the 300DR is more than capable in their eyes. Ignore the 90w RMS capability (I have little clue what he means), they pointed me to the peak power capability as sufficient, which @Innocent_Bystander called out earlier. Will you get a better drive out of other more powerful on paper amps, yes. Would statement be a fit yes BUT power doesn’t seem to be the issue for me if in demo these worked out but not at home, and in their experience this is a good match.
Second - For sure source first IS the recommended approach but I am not showing symptoms of a source issue in their eyes. For sure I would have an amazing experience with a 555PS on the NDX2 - greater clarity, separation and grip but it won’t put more depth to the bass in their view.
Third - The room - this is where Naim were really curious. The engineer immediately asked what had changed environmentally given the symptoms I was calling out. Interestingly the quietest voice on here was where he went to - plinth or feet, and floor type. @NickofWimbledon and @Bluesfan seem to be genius according to the engineer. He said that the cheapest first thing to do is isolate properly - he said I would be surprised.
Fourth - The room - height seems to be of real interest to the engineer and we need to hire a professional to come look before spending huge amounts on front end. Interesting. @edanmars they suggested the same as you. The height could impact the drop off hugely.
Fith - The room - I mentioned the one out of the box recomendation of @Buns that @HungryHalibut asked to be explained by @Innocent_Bystander . Bass traps behind the speakers. The engineer went silent and I could almost hear him smile… he responded ‘very, very clever’. Yes this is apparently a good idea.
Lastly - he said that the new classic NAP350s would be his choice on this set up and he has demoed them a number of times in this set up.
Anyway, a LOT learned and it turns out you are all right individually to some extent but at the same time there a many rights…
One path I am not following is the sub route - thanks for the recommendation. The Naim engineer said he thought the 350s were better than the 500 with these speakers (of course he would).
No mention of the 252? It strikes me that that and the room are the two key differences between you being happy at the dealers and not so happy at home
I also wonder if the speakers take some time to come on song - were they brand new or demo or used? I have Kudos 808s and they needed 200 hours before I was sure the brand new demo pair I had at home were the speakers for me
It seems that getting these B&W’s to work is more complicated than getting world peace.
THIS. Yes this is my conclusion. I am a complexity scientist and a psychologist specialising in system dynamics so I am used to synthesis of the complex. I love to get multiple perspectives - its is in fact the only way to find your own truth… as long as you can make sense of it all… I fully agree with your list and the order AND I do not dismiss the amplification point you are making. Maybe a little down the road.
Funny the 252 was only mentioned here - neither Naim nor B&W picked that up as a biggie. It isn’t that they didn’t acknowledge it. They said in the end the 282 will need shifting. TBH I am thinking of moving the new classic in the next 2 years so that will solve that issue.
I would do bass traps and duffision, absorption first. After that I would go the power amp route. I do not believe there is insufficient bass in the NDX or the pre as they stand. But off course have no idea. I just imagine the NDX and 282 with dual Hicaps would be sufficient.
I’m not sure that ‘of course he would’, given that the 350s are half the price of a 500. If you are thinking of the NC route, then the 555PS, 252.etc isn’t really relevant, and a 333 and 332 make more sense.
Do t forget that you were asking Naim to focus on the bass issue. As we all know, or hopefully do, there is a lot more to good sound than bass. The soul and engagement are what you get from a carefully thought out and balanced system, so nothing Naim said negates the need to get electronics up to the speakers. But as you said, you have time to work on it.
I do have time and am enjoying the journey.
One thing I have learned on this journey we call life is to listen to others with compassion and interest. There is so many other experiences we can learn from. At the same time to take this information as only that; their experience based on their own views and context.
There are facts but often the facts get lost in the opinions and the complexity of it all.
The ‘of course he would’ was a nod to the fact that he new I wasnt headed down a 500 route and so the NC 300 series was an obvious choice; I doubt he is going to say, ‘actually our amp can’t handle it go look at Michi or Chord’. 
I only asked about bass as this is the only element out of balance - having re connected with my dealer he is going to bring a 555ps and the 252 over to play with in the new year once I have had a look at the room and the feet.
NC is a temptation but requires a big overhaul and one off investment…
S
That sounds like a great plan. I noticed above that you are a complexity scientist. Could you explain what that is, as it’s a new one on me. It makes a great change from people saying ‘I’m an engineer’ as if it makes what they are going to say super important.
Yes, it’s better with nap 300 on tweeters.
A summary of your experiment would be interesting. Good listening.
“Duffision” 
. Difussion
Or even Diffusion… (I do like the sound of Duffision though!)
Thing is, the issues and matters you are playing with here aren’t binary, they are qualitative in the most part, and, to make matters worse when expressing to others, highly subjective around one’s personal taste in replay, also borne from the type & genre of music you like.
It’s akin to satisficer and maximiser behaviours in economics – it’s simply a question of how far you are prepared to go towards your goal.
It’ll be interesting what differences you think a 555PS(DR?) and 252/SupercapDR make to the system. IME, adding a PS (esp a 555PS), to Naim source kit generates a much better presentation, including more apparent but better controlled bass.
The Naim convention is to put the better amp (better quality, irrespective of power capability) on the mid or mid-top, because that is where clarity shows best. However given that it is the bass you are trying to improve, the more powerful amp should be on bass. (In professional audio, active driving is invariably based on providing the power needed, with much more on bass than mid, more on mid than top, though in such usage the norm is to use similar sound quality amps so the primary difference is the power capability, as opposed to with amps like Naim’s the primary difference tends to be sound quality and often only minor differences in poser capability.)