a lot of things can cause this...but you can use the effects send/return and line/preamp out jacks on the back panel to narrow down where the problem may lie.
test the power amp by plugging your guitar cable directly into the effects return jacks....i think you have pre and post effects return jacks. what does the output sound like?
next, plug your bass into the input jack, and plug a second guitar cord into the line out or effects send jack and connect the other end of that 2nd cable to another amp. make sure the volume of the 2nd amp is set low...this way you can test the preamp of your hartke, to see if all the front panel controls are working ok.