I'm sure Mejames is right about the 750's, and the comparison is a little unfair not only because of the added power, but also because the Reference amps have an improved input amplifier circuit topology and power supply, not to mention the better parts. Upgrading the coupling caps in my 185's is probably the next step, although going to 250's or 450's could also be in the picture eventually (the Ref amps are too big and too much money for me - and besides, I like to think I can and should be happy without having to have the best :-)
The tube swap to KT-88's requires nothing more than installing the new tubes and biasing as normal. (This is not the case for KT-90's, which need resistor changes.) The driver 6350's are the same as always (there's not much to choose from with this tube, NOS being tough to come by and limited to just a couple of options I believe). I've played with several different input tube varieties of 12AT7's - maybe I'll do a thread on the results sometime.
About your comment that "small-scale" music shouldn't present very taxing power demands, just today I did some comparisons between tetrode and triode using Sonny Rollins' "Way Out West" MFSL CD. Jazz doesn't get much more intimate than this: just a sax, bass fiddle, and a small trap set with a little percussion. No chords at all, and the tunes are taken at easy tempos and volumes, with plenty of space left in between the notes. Still, there are differences to be heard when listening between the two modes - even at moderate levels - that could relate to power, such as bass control and dynamic range. As ususal, those catagories go to tetrode, while overall naturalness of timbre, particularly from the low midrange up, goes to triode. Which presentation sounds more 'real' is a bit of a toss-up, depending on what sonic qualities you prioritize.
I think that to approximate reproducing the true sound of any acoustic instrument or voice, no matter how delicate the music or spare the arrangement, still presents an enormous challenge to any power amplifier, and I tend to agree with VTL's position that a lot of the inevitable compromise is simply related to a lack of available ultimate power. The task, of course, is to make a power amp that can deliver the needed level of clean power and yet do it instantaneously and with good musicality. I guess VTL probably does as good a job of trying to give us this as anybody, but there's a price to paid both monetarily and in size/weight/heat/appearance, and besides most speakers can't take full advantage of dynamically unlimited amps anyway. Most audiophiles will never attempt this, and the SET/horns crowd would pooh-pooh the notion, but my suspicion is that a home 2-channel system utilizing several thousand watts of multi-amped power would not necessarily be overkill strictly from a sonic accuracy standpoint. (You here Sean?... :-)
