Tube amp problems: No sound unless I dime the channel or master volume.

[p4vl]

I don't know what I did to my Engl Screamer head but all day, it won't make a sound unless I turn the channel volume or master volume all the way up, then back down to a normal level. Even if I put the amp on standby, I have to crank one of the volume knobs to get sound, then turn it down to acceptable levels.

The amp is a 4 channel (technically 2) amp and this has been happening on the clean and lead channels.

I was planning on taking some photos of the amp today so I could sell it.