Need help Troubleshooting

I had a system installed @ Tweeter in Rockville about a year ago no issue until now. My Alpine MRD-M605 amp is now acting up. The subs it is running only want to play when the deck is cranked up all of the way. It has a digital display which is supposed to produce an error code if anything is wrong but I get nothing. The voltage is reading at 13.6-13.7 and I assume it has a signal since it can play at full volume.

Willing to pay someone to come help diagnose the issue.

Thanks,
Russ