Tube roll

KT-90's have a little more power than KT-88's or 6550's. They might have a little (very little) tighter base, but do not have the richness in the midrange that the other tubes have, IMO. I have a pair of MFA-M200 mono amps and have rolled KT-88's and various kinds of 6550's. I recently put in RAM 6550's which are matched by RAM Labs and are actually Sovtek 6550 WD's. These are the best tubes that I have tried on these amps.

Hello Markus, I've rolled many but not the KT90s, ie KT88 Gold Lions reissue - very good overall, Tesla NOS KT88 (NOT new production JJ Tesla)- excellent overall but tubes are hard to find, 6550s SED - good, Tungsol 5881 NOS - a great tube but lacks bottom end in this amp, Sylvania 5932 NOS - excellent all around - my personal favorite - good tight bass - excels in all other areas.
A great amp. I was using them on my rear and center channel but they deserve better so I,ve put them up for sale. Don't forget about the imput and driver stage for rolling. Enjoy. Bill