I am looking to retube my Ampeg SVT II. Noticed my bass signal was dropping out, the other day. Could be a preamp tube, but I am not sure. I will have a tech check it out. In any event, if I had to retube, I was thinking of dropping in KT88s, instead of 6550s. I am pretty sure they will fit, but maybe you other SVT II owners could verify that? I also have a few KT88s lying around, that were in my old Trace Elliot V6, and some other Sovtek KT88s. There was really nothing wrong with them, when they were in the other amps. Can they me put in, even though they are not a matched sextet?