You can do a small fix right away and that's switching from bridged to single channel. Looking at your amp, just take the - lead off the far right side and move it to right next to where the + lead is connected. See if this keeps your subs from shutting off. If it still keeps cutting off, you need to look into a new amp.
---------- Post added at 07:31 PM ---------- Previous post was at 07:30 PM ----------
Easiest sign of a blown amp is the capacitors. If they're not smooth on top or there's goop you can't explain formed around the internals, it's blown. Will also smell unpleasant when you pull off the top casing.