I don't suppose the amp has a clipping indicator on it? That's a very useful feature which you'll find on any pro amp, but not on most home or car amps. Peak-reading level meters for the line levels and power meters on the outputs would be helpful (with a peak level memory to catch fast peaks). Someone must make that kind of thing (Radio Shack?). You could use a laptop with level meter software if you calibrated the levels with a voltmeter.
There's a kit power meter that goes up to 2000 watts at 4 ohms. Google for k4307 audio power meter; it sells for about $40 US. The manual [URL="http://www.vellemanusa.com/downloads/0/manual_k4307.pdf"] has a schematic if you wanted to build one from scratch.
I think the wise thing to do is add more subs: this has several benefits:
1) less power per sub, which would reduce the power compression caused by voice coils heating up.
2) multiple subs are more efficient (+3dB for every doubling of number of subs if total power is constant)