With the exception of the current JBL L8400P many JBL/Infinity amps have had difficulty.
You can take off the screws of the plate amp. You may have to pry it out (sometimes the are glued down for a better air seal). You can look for any discolored parts (sometimes as capacitors age, they will swell in size). If there is nothing obviously wrong, then you may need the service manual, a volt/ohm meter, and soldering equipment. Sometimes, you can do a large number of Google searches to locate a site with the manuals (that usually cost $15-25).
Subwoofer repairs typically cost $200-300 with a 30-90 day guarantee. You would also need to pay one-way shipping to the repair location.
You can get replacement amps at Parts Express and OAudio but may have difficulty getting one that fits.
At around $200, individuals like the Premier Acoustic PA-120 and BIC F12; at under $300, individuals like the BIC PL-200. You need to check the respective forums for how to get the best price.