Hi Ecclectique:
Thanks for your advice on the tubes - your advice is encouraging, as it's the same advice KrAudioProducts gave me too :-) - I'll be getting a matched quad of cryogenically frozen KR 842vhd tubes this week.
I don't have the full story on how/why the amps were modded , but my understanding is that they were done so to accept a tube like the 300bxls tubes, rather than the previous tube that the amps came with (which apparently only lasted a few hundred hours, proving to be very expensive to replace on any ongoing basis). Also, the amps originally were built with a transducer (not sure what this is really), but apparently that was a bit of a design mistake, so the mod took care of that too. All told, the mods are a bit of a puzzle to me.
Fingers crossed the amps will sing with the new tubes. I'm also hopeful that the power output of the tubes will be able to drive my Virgos well. it's a concern I've always had given that the Virgos are 89db efficiency and operate at 4ohm load. I've been tempted to get more power, although these KR amps do sound sweeter than my old Blue Circle BC2 hybrid monos which were supposedly 65wpc.