Well as soon as it powers up, it starts to oscillate on one frequency whether or not there is an instrument plugged into it or not. And it doesn't change when the instrument is played. Pretty simple set-up. Uses a simple linear power supply and an op amp as an output device.

Could it be a bad input R? I had a similar thing happen with another amp when an input R was broken.

Any other thoughts?

It must be a solid state bass guitar amplifier?

Yes. It uses a op-amp IC as the output device.

Something may be screwed in the feedback loop.

Alot of times if you know what model the OPAMP is you can look up the feedback circuit diagram.... If you can, check the parts and make sure it still works, without proper feedback on those, they will oscillate uncontrollably...