Peavey low output

[felix_kentish]

Hi,
I've just found this forum while searching the internet for a similar problem.
I have a Peavey Combo300 amp/sub combo.
I lent it to a mate and i think he put the output of a preamp-equalizer into the low gain input of the amp.
Since i've had it back it just doesn't have the output it used to.
All of the equalizers on the amp still work and it still sounds good, just very low output.

Any idea's where to start?

Thanks