The last time I had a headunit professionally installed they connected the remote turn-on lead to one of the front speaker leads, and after fixing it, cited that it was a common mistake because "both wires are similar shades of blue."
So because of that possibility, I'd check out the wiring behind the head unit, because my guess is that your amp is not even getting a turn-on signal. If something is mis-wired there, then you are going to want to check the head unit and amp fuses as well (I couldn't tell if you said you did that already). Hopefully something there works out for you, good luck