how to troubleshoot sub/amp combo

HI Can someone point me in the right direction? My Sub/amp combo was acting weird lately with it cutting in and out After a while I wasn't able to get any power out of the amp so replaced it with a similar model. After hooking everything up, the new amp is getting power, however when connected to the sub, there is no sound? Anybody have any ideas on how I should begin troubleshooting? Remote is good as well as the power going in

I don't have ready access to a multimeter. The fuse appears fine also as another amp also shares the same power connection. How would I check a sub with a battery? Would I just use the stadar D Cell or is it a slightly different way?

A standard D cell will work fine.  Touch and hold the positive speaker wire to the positive terminal of the battery.  Then touch the negative wire to the negative terminal of the battery.  At this point your speaker should push outward.  You will hear it also.