This amp is 2ohm stable, but if you use a dual sub setup and use both sets of output terminals it is automatically wired parallel at 2 ohms if you then have 2 subs that are dual voice coil and the voice coils of both are wired in parallel then the load become .5-1 ohm and the amp is unstable (solution) wire 2 dual voice coil subs individually one from each terminal, and then wire the voice coils in series rather than parallel

With your multimeter set to DC volts, the black meter lead on the ground terminal of the amp (not on the point where the ground wire connected to the vehicle), touch the red lead alternately to the B+ and remote terminals of the amp. If the voltage drops below ~11 volts on either terminal when you drive the amp hard, you need to check the wiring feeding whichever line is dropping too low.