At the point of making a final decision now. In rereading this discussion, I was struck by jmcgrogan's early comment that
"I tend to agree with George, I'm not sure that a tube amp would work well with the Sonus Faber Olympica III's, they are not an easy load.
Any VAC amp that can put out 100 wpc and use KT-150 tubes though MAY be able to do it.
I was actually using a VAC 160iSE integrated amp before the XA-100.5 mono-blocks, and my speakers have similar specs (90 dB, 5 ohm versus your 90 dB, 4 ohm).
Switching to the XA-100.5 amps, the sound-stage opened up immensely. Not really surprising going from a tube stereo amp to SS mono-block amps.
Mating the Pass amps with an ARC Ref 5 preamp, the resolution of the highs and lows also became more extended and defined.
The only place I would give the edge to the VAC was in the midrange.
Though the Pass are quite warm and musical, the VAC had that touch of *magic* in the midrange, that fuzz on the peach quality, that only a tube amp can do."
I would suspect the sound from the upper level PHI power amp would improve over the VAC integrated that you had, possibly not in a huge leap, though. And it sounds like your satisfaction was clearly greater with the the Pass 100.5's, excepting the extra touch of tube magic in the midrange.
Going into this, I had really been leaning toward the 100.5's, and as I noted before, had been on a waiting list for a pair at Reno HiFi for the past year. I love the look of those amps, and would have bought an available pair (had one shown up) except for the fact that someone whose knowledge I truly trust, and who has owned and enjoyed about half the Pass line of power amps said they were his least favorite because the bass was puffy and inadequate.