I control two amplifiers (Kicker ZX400.1 and Pioneer GM4300F) with the M2-ATX-HV's J6 connector (just the 'REM" pin, don't use the "GND" pin). been using it for months, no problems here.
most amp turn-on leads draw less than 50mA so for
most amps the J6 connector should be able to provide plenty of power for it. I use the "-HV" version of the M2-ATX so maybe it was an issue with the original versions?
as for the connection, I soldered the wire directly onto the "REM" pin (I ran the wire to a 1/4A fuse, then to a I/O connector on the back of the PC case).
if you don't want to solder, then there are a few connectors that should work. the J6 connector itself is the same as a 2-pin PC fan connector (these aren't used much anymore, most modern PC fans use 3-pin connectors). the pin spacing is pretty standard so there are a variety of PC connectors that you can use; any PC fan connector (2-pin, 3-pin, or 4-pin) should work, PC case header connectors, USB header connectors, Audio header connectors, floppy drive power connectors, etc. if you have any old PC parts lying around then chances are you have some old component that you can cut the connector off and use.
good luck