My amp wont turn on
The reason it used to cut-off at 60% volume as you said before,
Is probably due to the "protection circuit mode" being tripped.
You can try resetting it by turning off your CD player ,removing the Batt( + ) and Gnd ( - ) terminals from the amp for a short period. Reconnect them Allow the amp to receive power but not switch on (keep cd player/remote power source off)
If you dont know how to wire the CD player to control the amps power status , the easiest way is to connect the 'power antenna' or similar low voltage throughput wire from the head-unit to the amp's Remote terminal.
My old amp did the same thing ,and would need to be powered off/on before resuming sound output, sometimes I had lots of trouble with the protect mode staying on , even with little or no sound output from headunit.