Amp stopped working

If the amp is on, but not making any sound, you may have a failure based on the tests you've described.  Work all of the switches and knobs in case resistance built-up over time.  

Using a DMM to verify voltage on the power and remote is step one.

Step 2 is to play a 60Hz test tone and measure the output terminals (no wires connected) with a DMM to look for voltage (VAC).  This lets you safely play with gain and settings without concern for damaging speakers/subs. 

If you measure voltage (more than 10VAC) than the issue is probably in the sub box wiring.

I've seen sub box terminals fail and lose connection.