the model will be something like GM-D****M but if its the one i'm thinking of, they only make about 400wrms or so, so maybe your sub is too large or your signal/earth/power is poor. Take it to a place with a test bench (ie Autobarn, any CAMS dealer, any independant car audio store) and get them to have a look. Now they will probably only be able to tell you whether its broken or not, and give you an idea on what you can do to replace it.
I usually just reccomend replacement for any amplifier worth under 500-1000 depending on what type, seeing as though professional electrical engineer labour doesnt come cheap, neither do replacement parts from pioneer, IF you can still get them. Other option is to send to pioneer, you'll pay a fee to get a quote to repair it (only paid if you dont get the job done) and then they'll fix it once you pay the quoted amount (be prepared for a long wait, they're understaffed, and non-warranty repairs take a back seat to warranty jobs).
so look to replace it, you can flog the broken amp (if it doesnt work) off to some douche (if you have no scruples), or use it as a boat anchor or whatever.