Why does my amp not produce a sound from the instrument's signal?

After thinking I found the problem, fixing it, and testing the amp, it still only makes a slight humming sound like most solid state amps. It gets quieter and louder with master controls, and equalization controls alter the pitch of the hum, though you can't hear any signal from the instrument at all even on the highest settings.

Please note that I have already inspected how the input jack and its cable's connection is; tight and sturdy. The solder on its contacts seemed to be a little poorly done, by whoever touched it last. I cleaned them and redid it myself to be positive the connections were good. Another thing to keep in mind is that changing the DSP control between 'bypass' and any active setting has no affect on the sound.

Also, I've gone through the main board several times with my multimeter and haven't been able to find any anomalies in components. The speaker wires themselves have been used to test the speaker on a separate, working amplifier, so I know they are in working condition.

At this point, I don't really know what to look for. Any tips/suggestions would be greatly appreciated!