Please don't take offense, but some tips on submitting problems on Fixya:
- Resist the urge to use all capital letters. Reserve them for frustrating yells :).
- Use a period or two. It's very hard to read without them.
The easier to read, the better responses you'll get!
Anyway, back to the issue at hand.....
Definitely get a handle on the gain adjustment. Both channels should start with a gain at the 50% point. Also, you can try bridging the amp ("+" speaker outputs tied together, "-" speaker outputs tied together) and run the full 800W into the single 10" sub, but that is going to depend on what your sub is rated to handle.
The only other thing I can recommend is call around to a couple stereo shops and see if anyone can test the amp for output power. That'll verify whether there's really a problem on the one channel.