Personally, I've never liked any of the Sunn stuff for guitar. They started as a bass amp that was very similar to a Dynaco Mk3, and even used Dynaco transformers. A couple values were changed in the preamp to make them work for guitar, but they're still kind of a hard and clean distortion. The EL34 amps would be better than the 6550/KT88 amps, but to me, their voltages are all too high to really sound nice for guitar. I've used my 200S bass amp in the studio for guitar on clean stuff, and it gives a different vibe than your typical Fenders, but I don't like it at all for natural amp distortion with a guitar.