Welcome back to the forum!
…reading through the comments I should add, Welcome back to the madness!
I ran with a bare NDX 2 for six months before adding a XPS DR and found it to be a huge uplift. (my system, my room, my ears)
Good luck upgrading your current system and look forward to seeing a picture over in the ‘System Picture’ thread sometime in the future…
Isn’t that bit strange if hiss is part of Naim products when they put more effort than anyone else in removing noise creating more blackness by separating power from signal electronics?
They would probably hiss even more otherwise 
My guess is that it all comes down to the basic circuit design. Obviously other designs have less hiss / bigger sound/noise ratio, but I suppose Naim thinks that theirs makes nicer music, and if it comes with a bit of unavoidable hiss, so be it. Kind of similar to not adding an inductance network to the power amp output to make it more cable agnostic, and not adding output relays to avoid a thump when turning on/off.
As I prefer music over silence, am willing to choose my speakers accordingly, and because I agree that the music from my Naims is great, this is fine with me. It’s probably not for everyone, though. (I don’t really get the quite many threads about “my amps hiss!?”, surely a dealer should educate the customers about it and enable them to make an educated choice).
And about the proverbial inky blackness: Like all the things we perceive, I would think that it is in large part not an absolute measurement, but defined by the contrast to something else. If the sounds are more lively and realistically energetic, we will hear the gaps between the sounds as more black than if everything is flat. I believe that this is more important than a difference in the absolute hiss level in dB, when even with Naim you can’t really hear it from the listening position, anyway. (Unless silly speakers, close range, or simply being sensitive to it / not liking it)
You would certainly hope so at double or triple the price. 
Adding an XPS DR to my bare 272 is one of my most memorable HiFi moments to date, and I expect it to remain so. I think the XPS DR is a little under rated.
I did 
And it stayed! But the uplift from bare to XPS DR was greater imo
I’d hold off any decisions based on 3 weeks of introducing any naim unit, especially if it is new, but even if it’s not. Give it another 3 weeks, it may have quite a bit more to give!
From my experiences (and humble opinions), the 555PS-DR is optimum for the NDS (and ND555) …the XPS-DR is optimum for the NDX2…I found the 555PS-DR on the NDX2 to be a little too much, both in sound and investment/cost…
The 555PSDR is superb and totally at home with the NDX2. Anyone who can afford a 300Dr can afford one. And if they can’t, their system building priorities are wrong.
The 555ps is an unbelievable upgrade to the ndx2… I upgraded to and EE switch and 555ps in the same week and the difference was staggering…
The XPS DR is good but miles away from the 555…
Everything is fuller and the best bit is it’s amazing at lower volumes…
Upgrades are usually quite minimal in my opinion… the supercap to a Supercap DR was very small… The XPS DR was good but not amazing…
Trust me the 555ps and the switch was game changing… I paid 5k for mine 2021 model never been used…A second hand one can be got for 4K ish… I’d forget the XPS to be honest… not worth it
Jeez, I guess we should all just send you money so you can buy our systems for us. Honestly sick of reading your factual opinions of late.
What, pray, is a factual opinion?
I think that’s a bit harsh…
all these upgrades are a fortune and you really need to do them only once… I’ve been saving for years to get my system and this forum helped loads for me. HH is bang on IMHO
Precisely…
Anyway…
I’m listening to the Hollies ‘The air that I breathe’
And it’s the b@ll@cks 
An Ndx2 with 555 dr on a bottleneck like the Supernait and mini 1k monitors is just not at home. All the total cost could be spent so much better.
For example an Ndx2 / Xpsdr with 282/ 200 and some 3 k floorstanders would be much better balanced.
I’m still on give it more time page … I had a bare ndx for many years and thought it was great. I bought that from new but it took a while to run in. I ran it with an 82 and a 52. I struggle with the idea that ndx2 can’t hold it’s own in the company of the amps being discussed without a separate psu (though I have to admit to not having heard the ndx2). Surely the 555ps is a fantastic upgrade to an already superb streamer rather £7k crutch to get it over the line.
Welcome. I’m loving my NDX2/XPS DR combo. Haven’t heard it with a PS 555 or without any PS so my opinion would be worthless. I can tell you that it’s a great DAC and streamer.
I’ll enter the fray…
So, I’ll humbly say the PS on the NDX2 is required before you go to the 300. The XPS DR will be very good and the 555PS better (but in my view only worth the cost if sourced second hand).
If budget doesn’t extend to the PS for the NDX2, I’d forgo the 300, get a 250 and XPS DR for probably less cost and a better balanced system as a result.