pioneer 1500spl subs

You need to set your gain on your amps correctly (they arn't volume controls they're level matchers). First turn the gain on both all the way down, then put in a cd in your head-unit and turn the volume to about 3/4 to 2/3 of the way up, next slowly turn the gain on both amps up until you hear a popping or buzzing noise coming from your subs (this means the amp is clipping and sending your subs a distorted signal), slowly turn the gain back down just before this point and you have your gains set. Since you have two amps you should try go get the setting on both to the same as possible. Don't turn your head-unit up past the point where you set your gains and don't boost the bass or use the bass boost (this will prevent your amp from clipping and potentially blowing your subs).

Sounds like from what you described you have two 4 ohm dual voice coil subs.  Each coil is 4 ohm, so the speaker can be wired to either 8 ohm or 2 ohm.  Use series wiring to achieve 8 ohm on each speaker.  (Combine the two subs in parallel to get an overall 4 ohm load.)  Running a subwoofer to an amp at an 8 ohm load won't hurt anything, but might be considered a waste of amp power for sub-frequencies.  What exactly are the sub specs?