Well, with my 42.5/180 I added a Hicap 2, very expensive and frankly not worth it.
The 42.5/180 works very well without a PSU.
I have had that 42.5 back to back with RPM's 62 and there is not much too choose between them, the 62 being a tad leaner, the 42.5 a tad warmer with more bounce.
mister6 has it about right with the power amps although I don't know the 110, something I must investigate.
I did also put an Avondale PSU in and didn't like the change of sound, I prefer the Hicap, more bass, boogie and bounce.
Currently I run a 72/Hi/160, the 72 definatley needs the Hicap to get the best from it.
All is subjective of course and there will always be trade offs.
For example the 42.5, which I highly rate has the Naim boogie and bounce but lacks the speed and detail of the 72, pop in the Hicap on the 72 and you get both.
The 250 being regulated adds another dimention; more weight, but slower than the 160 and 180 from my experience but a fine amp all the same.
The Naim game is a merry go round, I hope this gives a little insight but enjoy the ride, also the differences are not that great, it's more about what you like and what suits your rig.