When I bought the amp, direct from Bob, he gave me the option to have him send me the KT-88's that he normally sends or the KT-120's; his new company didn't come online 'til a short time later. Anyhow, I ended up having him send me the standard tubes and I ordered 12 KT-120's from another source. Once getting the amps home, I plugged in the KT-120's and they have been in pretty much the whole time. However, I did once after several months, try the KT-88's. The sound was actually a little warmer, but the tubes seem to be working very hard. What I mean is, I like to play my music relatively loud at times and with the KT-88's it looked like a fire storm raging in the tubes as the music played. These amps was/are my only experience with tube equipment, so that was kinda scary to witness. Maybe thats how power tubes regularly behave under serious music listening, but with the KT-120's the peak "flaming" is much less and conversely less stressful to see when enjoying your tunes. The only drawback with the KT-120's, is that you can barley see that the tubes are lit, even at night...and as we all know that's part of the appeal of having tubes...seeing the glow lol. As far as the warmth goes, I added some Mullard tubes (4004 & 4024) to the equation and that warmed and smoothed out the sound again.
Thanks for the info on the KT-150's. I think I'll stay with the KT-120's for now. My next move would be the upgrades on the gain/attenuator & 2caps on each amp; which Dracula1 spoke about earlier in this thread.