Frank, on my ARC tube amp...which also uses 6550's, IF I leave the tube cage on and have the amp on for more than an hour, I have noticed that the amp tends to misbehave...the tubes get a little noisy. I was told by my tech that it is best to have the 6550's get as much ventilation as possible. I think ARC runs these tubes fairly hot and this can quickly lead to over-heating problems. Even though you are using the tube in the preamp stage, I suspect the 6550's are giving off a lot of heat....NOT a good thing in a tight enclosed space. OTOH, as rsorren1 stated, best to work with your dealer, as this could well be a faulty unit.