ARC does not recommend KT88's but I'm not sure why. I've seen people try them with no problems. I had a VT100mkII and its an outstanding amp. I felt it was a little "dry" sounding for a tube amp so I suspect the KT88's might warm it up slightly over the 6550's. On a related note, my CJ amp is inherently warm and sweet with the 6550's and the KT88's didn't sound as good in the CJ amp.... It's all about personal preferences and this is what I found.
