Disconnect speaker cables and RCA cables, let it start up with only B+, gnd and remote connected, then check the voltage on amplifier power terminals - if all seems to be OK with voltages, you will have to get amp repaired.
Do NOT ever mount this particular amplifier on the subwoofer box! Well, this is cheap (good enough for that money) amplifier, main PCB is not enough secured from vibrations, sooner or later the the transistors, regulators and rectifier legs will broke off the board. Also the weak point of this...