With most amps KT88s for me have 'hollow' mids, maybe since I'm used to EL34 Marshalls. But, owning some VHT amps changed my mind on that. Those are biased right where they should be, 50+ ma and sound nice. Same with popping 6550s in there, or KT 90s (even better IMO). When you play a Marshall with 6550s (like my 83 2203) they can sound cold or sterile and I think that's because they are biased like an EL34, since the transformers aren't designed for 6550s you have to bias them like a 34 and they end up cold. The exception are when a Marshall has GE 6550s which to me are very 3D-like, similar to what Sylvania 6L6s do for Mesa Marks. But if you try and bias the 6550s/KT88s where they should be in a Marshall (50+ma) you end up stressing the transformers. Ideally, if you love Marshalls and KT tubes you could buy some custom transformers designed for KT tube draw and then you'd be all good to bias them where they should be.