I have recently installed a set of the 120s in 2 of my amps... First in a McShane modded Citation 2 with a level 2 plus kit and a complete rebuild...and secondly in a pair of Heathkit W6M mono blocks. I can hear a marginal difference in the Citation although I expected a little more out of them, but there is a marked improvement in the W6s which actually outperform the Citation now. I must add that the W6s have also had a McShane type upgrade to the power supplies and have more than 5 times the capacitance of originals. I'm sure this helped. I bumped the bias up a bit, but am still running them at a very conservative figure for the bias. I'm looking for longevity rather than the absolute max the tubes will do, but they do kick butt!
The only thign you need to be concerned with is 1. The height of the tube is considerably more than KT88s and 2. The filament draw is quite a bit more than 6550s or KT88s. Check your schematic and make sure you have the extra current to handle them....otherwise you might make a boat anchor out of your power transformer. An easy way to check if you don't have specs is to measure the filament voltage with the stock tubes installed. Then try it with the KT120s. If you notice a drop in the voltage your transformer won't handle them....don't risk it. Otherwise, have fun... they have excellent bottom end and are very clean sounding tubes. My favorites have always been winged C 6550s, but I might have found a new favorite....and the prices on them are dropping considerably.
Hope this helps someone.
