1 channel quiet in a multiamp

Hi everyone,
i have a Marantz MM9000 5 channel amp that was running fine until just the other day. one of the channels has become significantly quieter than the rest. i have checked to make sure i had the gains turned up the same, and i have tried changing the inputs from the pre to make sure it wasn't a cable/speaker/pre problem.

anyone have any ideas what may be the cause? there is still sound coming out of that channel, but it is much quieter.

thanks.