By the way did some serious listening last weekend to both sets of tubes (KT120 & 6550C). KT120 TungSol & 6550C Svetlana Electron Devices.

These were my findings:

Both sets of tubes have a magnificent soundstage, 3D at its best no doubt! However, the one thing I noticed was the playback levels. The 6550's soundstage was all neatly in place and sounded just right at any given level wihtout too much added punch. Altering volume levels did not impact the scale immensely, hence remained the same throughout.
The KT120 is a bit laid back at very low volumes, turn it up a bit and it's a whole new level of realism!

Wow I really enjoyed this whole discussion and pardon my ignorance as I'm just getting back into a system, and I just picked up a pair of the VTL MB-450s the first gen style built around 2010 from a nice lady that never pushed them out of tetrode, however she had her tube head friend put in a fresh set of the KT-120's And kept them biased and loved the sound of them. Well today I called VTL to see if there were any upgrades available for the amps and mentioned to Bea(I think was her name, nice lady) that I had the KT-120's and it went over a bit like a fart in church,she said that they don't recommend those etc. As these are brand new to me,just got them today and haven't had time to set up just wanted to make sure that I'm not going to damage these Amps in any way by running them, as also I don't have the cash to grab 16 good 6550's to swap out at the moment anyways. So advice? Thanks