Marshall MG100HDFX low on volume.

[Prairie Dawg]

I've got a Marshall MG100 etc etc solid state amp head here, and it appears to be pretty low on volume. Running it through a 12" Celestion it won't even drive me out of my laboratory, which is the size of a medium bathroom. The guy's story was that he ran it with the speaker disconnected and after that it had nothing. Well, it does have something, just not enough of it. I figure it should blow me out of my sneakers at close range, right?

It does not have any signs of heat distress or bad smells, water damage, signs of abuse, etc etc it just doesn't have any muscle or punch. I would hazard a guess that all it's got is about 20w on the output side.

Any ideas? Obvious things to look for? Places to start? Problem areas? Am I doing something really wrong running it through a single 12" Celestion? leaving aside the fact that it's a pretty sorry looking thing to carry the Marshall name, but nevermind-let's not go there.