What is it?

[Hotrod]

Hey - I have a quick question. My amp head started "cutting out" a while back. It mainly did it on the clean channel, but does do it on both. The sound didn't drop out completely, but rather that fuzzy sound it makes when someone hits standby, but is still playing. I put new power tubes in and had it biased. The problem is still there, but only seems to do it at rehearsal volume, not bedroom volume. Would pre amp tubes be a likely solution, or could it be more hosed then that.

Thanks