mono pre out handicapping amp?

Hello All, Just finished installing my latest system. 2 Sundown SA 10s dual 4 ohm wired to 1 ohm on an audiopipe apsm 1500. (1500 Watts RMS @ 1 Ohm) Set the gain with my DMM to 38 volts but the output is less than I anticipated. Box was custom built. 1.4 cubes per chamber ported and tuned to 36hz. Only thing I can figure is the head unit pre out for the sub is mono (1 jack). Would a Y splitter increase my output? Don't get me wrong it hits decent, but I've had systems in the past that were capable of being much louder with close to 1500 watts. Thanks in advance for any help.

Most amplifiers require both RCA inputs to be fed signal. A Y adapter will probably do you some good.