please tell me if my amp is broken...?

ocums razor......It's usually the simplest answer. Just start back from the begining. Your first conection. Go through your system as though you are hooking it up again. I don't mean to tear everything apart. I just mean to trace your path of installation. If you discover that everything is kosher than move onto the next thing. The amp would be my guess. If you have a multimeter than break it out. It's an installers best friend. See if your amp is even producing. You can even try things like checking for heat. If the amp isn't warm than it's not working. If it burns your hand than get the fuse out of it and get rid of it. Then follow thru to your speaker. Make sure you don't have any breaks in the speaker wire. Then make sure your hooked up right. Just be patient and diligent. Good luck

check all of the fuses to see if any are blown or not replace the one that are

and check the grounds it has to be a good ground with bare metal no paint at all

try the audio adjustments on the amp

and i also hear of some subs not working if the wires are hooked up backwords

Sounds like the input of the amp is bad. Just try connecting one RCA to the amp.

It sounds to me such as you have got a venture with you guitar, no longer you amp. If the piano sounds wonderful, yet no longer the guitar, then it is the guitar. The output jack could be loose.