AFAIK, Krell is DC coupled. If your preamp puts out any DC at all it can cause a problem. The solution is really simple, put a cap in series. 2 to 4uF should do. Get as fancy as you'd like. :)
Audio research and krell
So here goes I have a arc ref 5se and I am using a krell Kas 300s and recently I noticed the krell was making some noise when powered off so I made some calls to krell and found out that the older krell amps are not recommended to be used with tubes I have never heard this .so has anyone here heard this .second question if that's the case what should I move to for a new amp I like the krell sound. Speakers are revel salon 1 transparent cables