I do get sound if I plug the bass into the receive side of the loop and the output jack to another speaker also works.  When I plug an instrument into the input I get very faint sound.  When I unplug from the loop in the rear its very loud.  Auxillary input works fine it seems. 

Tried that thank you

Hello, I am trying to figure out an issue with a new to me rumble 500..I bought it as is hoping it is something semi simple.  It fires up and thumps powerfully when shutting it down.  No sound from speakers...faint sound from external jack into another cab.  The effects loop seems ok as i can plug into that and get sound...low volume unless I put a pedal to it.  It seemed to get a little better after I plugged headphones in and out of it.....im leaning toward input jack but any help is greatly appreciated.

Jim