Right this may not be the problem but you could have fried your whole system with that earth. That is by no means an ideal place to put it it should be bolted to a rubbed down hole in the spare wheel bay. Also what power is the amp because i doubt that 8 gauge wiring is ideal.
When the amp turns on what colour is the light.
Have you checked every fuse possible ie. Under dash, the one on the power cable to amp and the one on the Hu.