Thanks guys
Tom, yes I did ask AR about KT-120s, they just said no amid answers to a few other questions. Jim McShane (tube seller) explained that the KT120s are too heavy to be mounted horizontally in VT100, but they are OK in Ref110 because its tubes are vertical.
McShane just emailed back recommending the KT-88 for sonic reasons. But warns no warranty on them when used in tube-finicky AR amps that originally came with 6550s.
I've read that the TS6550 are more bass, thicker sound, while GL KT88 are clearer midrange, refined treble, more forward etc. One guy compained it's not enough bass in his PrimaLuna with small OPT. I'm sure they both have good treble and bass, but maybe one is 'richer' while the other is 'clearer.'