hey, I have an old AIMS head that I bought from my friend for 100 bucks, it's a killer head but I was there since he first bought it (2 and a half years ago) and the tubes I'm pretty sure have finally gone. It runs on two 6550's and it has two ruby 6550 tubes in it right now. I want to retube it with either 6550 OR (this is what I want most to do) KT88's. I was thinking two JJ/Tesla KT88's. I use it for bass and guitar and I was wanting to know if the KT88's are a straight drop in replacement and if it's possible to do a rebias myself because I don't want to pay amp tech prices to do it. Thanks!