4 channel amplifier audiobank

Hi all,

I have a 4 channel amplifier, which has low output on the 3rd channel. there are no shorted components.
Each channel has 2 N-channel mosfets - FQP33N10. I haven't come across an amplifier like this before.
could it be a pre-amp issue or dirty switches?
No, i haven't done, will do so

Pic below: