Setting my amp gain with a dmm

Multiply the impedance of your subs by the RMS wattage of your amp (4*150). Then take the square root of that and you get the voltage reading you want (24.49 volts). Then turn the gain on your amp all the way down, disconnect your sub from the amp, connect the DMM to the speaker outputs of your amp, put in a test tone of 50 hz, and turn up your cd player to about 75% of its maximum. Just turn your gain up slowly until you get the above voltage reading (or close too it). This isn't the exact place for your gain, it might need some fine tuning but its a pretty good guidline.