Great question Graham. It throws the following into my mind:
I know why I prefer the former in each case but I’ll leave others to take this bait. 
. Although I would be very interested in opinions. In my mind the former in each case has a very visual argument (as opposed to a scientific one) in terms of the physical layout of the internals as to why some feel they are “better.
I am always intrigued, Paul, when I read of people such as yourself preferring the 52PS to the Supercap. It’s a comment that I’ve seen expressed a number of times over the years, and I always wonder why that should be. When I bought my own NAC52, all those years ago, the 52PS had already been discontinued, so the Supercap was the only option.
Two questions.
First, what are the internal difference between the 52PS and the Supercap?
Second, why did Naim make the change?
Does anyone remember that far back?
In other words it is a monoblock (and the 250 isn’t a “dual monoblock’ with two 135s in one box), which is widely recognised as a beneficial approach for sound quality. I wonder why Naim hasn’t continued to offer the 135, updated in the same way as the 250. Can the 300 and 500 can each be used with 2 power supplies, one per channel, creating the electrical equivalent of monoblocks?
Yes the 300 is effectively two mono amps sharing a single case and single power supply (and fan control board), albeit in the two box head unit and PSU configuration. IIRC, the two power supply approach (single head unit with two power supplies) has been tried at the factory (with a NAP500 ?) with variable results. It’s certainly been mentioned on here in the past.
The NAP S1 route is the way to go if you want true mono block operation 
Naim found performance advantages with using one extra large toroidal transformer over two smaller independent ones of similar overall cost. Of course you could use two of those same extra large transformers, and we listened to a NAP500 powered by a pair of 500PS units. It did some things that initially sounded rather good - there was this huge airy soundstage - however, it quickly became apparent that this was more of an artefact and that the cohesion of the music was being pulled apart and was more a hifi “effect” than music. IIRC, Roy speculated that it may be down to “parasitic effects” of the unused sides of the power supplies. This would possibly be much improved by designing a dedicated mono supply, but again, we’re back to the findings at the start of this paragraph…
Thanks Richard 
I would hate to think that there may be parasites breeding inside my 135s. I shall ask my dealer to instruct Naim to give the internals a strong dose of insecticide when they go back to Salisbury for servicing.
I think the reality is that listening to my system these days I am completely satisfied with the sound. I suppose I prefer the 52ps because I don’t feel it’s lacking in any way. I’d do like the beefier internals but that’s not a scientific reason for saying it sounds better than an sc. I could probably just as easily have ended up with a sc all those years ago but my 52 was what was out there when I was looking to buy. My only conundrum now is whether to put an XPS on my NDX. Every time I think I’ve decided to I listen to it and come to the conclusion that it doesn’t need it. I think I might actually have found my perfect set up after all these years. 
We may be getting off-topic, but the PS options are interesting to me.
The biggest problem with the 52PS is that they are almost never sold in isolation, so I don’t see how I could get one. The alternative of 52 + SCDR hadn’t occurred to me until I heard 52 + 300DR. However, I doubt my old ears will get too excited about the difference from swapping PS anyway.
I don’t know about the advantages of splitting your 2 boxes into control and power (a la 300) or 2 monoblocks (a la 135), though @Richard.Dane 's comments are as usual informative. However, I can say that I had heard a 300 and not been at all enthusiastic (largely because it wasn’t enthusiastic or engaging). I found 300DR to be a very different beast - I bought one. Had I already got 135s, the gap versus 300DR might have been more ‘different’ and less ‘better’, so which is best will be driven by room, speakers and personal preferences.
As for @graham55 ‘s other question - 52 versus 252, isn’t that the easiest choice of the lot, however the components’ differences explain the sound differences?
They pop up on eBay now and again. There was one a couple of weeks back. Can’t remember what it went for. I’m sure some people are still “upgrading” to the Supercap even now.
Be careful here. Some of us within Naim used old 52PS units converted into what were essentially super-Hicaps (great on a Prefix). IIRC the Burndy feeds were disconnected so you can’t use one of these on a NAC52. There are doubtless a few of these “out in the wild”, so to speak.
Thanks Richard - that’s another reason to tread carefully.
Can I just check here, please, Richard, that you were using an entire 52PS to power a Prefix? Did this result in styluses (styli?) leaping out of the LP grooves or other forms of mayhem?
Well, I suppose, why not, if you can…
Presumably this could be re-engineered back to stock relatively easily? Assuming it was just that the Burndy wires were disconnected.
Probably, yes.
As said. They use the same regulator-board and amp-board as the 250 but they are a) mono-amps with larger heatsinks and separate everything, filter caps etc. and b) they get a much more powerful drive for the regulator-board. They are also more practical and easy to place than most Naim-amps. Just simple input, speaker output and power. No external PS, no burndy, no snaic. But this also goes for the 250.
And if you want to use with non-naim preamp just make sure it is a modern one with enough drive. I’m running mine via the Lundahl-transformers in my preamp and with rescued Naim Lavender interconnect cables terminated with Eichmann Bullets (RCAs).
When I finally bought new speakers this year I tried other amps that got good reviews with those speakers but finally always went back to the 135s. I also prefer the 135s on a simple wooden rack (bamboo) over the complex Naim Fraim or on an isoAcoustic Delos platform.
With digital a very good source pays-off.
Interesting - thanks Richard
The 180 looks to have similar components to a 140. That cap/regulator board top left is out of a 140. But, the major ( perhaps only?) difference being a decent sized transformer in the 180 over the 140. Plus the bigger case.
Yep - that about sums it up.