GXR10 help

ok my buddy just got 2 polk GXR10s with a RF 201s amp. when we have both the subs hooked up in stereo they won't pump at all, but if we unplug one of the channels (either) the sub still connected will pump at least 3x louder than before. we know its not a problem with the subs (we hooked them up to my 600w kenwood and they pumped fine) and its not a problem with the deck or amp either cause we hooked my 12" kicker comps up to them and they pumped fine. we tried wiring them in parallel and bridging the amp but that was much worse than in stereo, we didn't try series wiring cause i thought that'd be way too big of a load on the amp. any ideas of what to do?