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