The crackling sounds as much like a crummy connection or solder joint as a bad tube. The power variation likewise, possibly more likely to be in power amp around the drive tubes ("around" them, not necessarily DUE TO the drive tubes)
Could be a jack, or something on a PWB.
Try the preamp through the other amp..... if it crackles, you know it's the pre, if not try the pre of the other amp through the power amp, and see if THAT does it.
Yes I USED TO work for Ampeg...but I haven't forgotten everything.