We have to get your whole connection scheme figured out first. Are you just running a passive sub (my guess since you said "standard speaker wire inputs") on the B (or 2, or remote, or whatever) output of your receiver with your mains on A?
If that's the case and you're getting low levels then you probably have an efficiency mismatch between the mains and the subs. If you have pre-out, main-in jumpers on your reciever then you could use a "Y" cable, run one output to another amp to run the sub and the other back to the main-in. Or, since you state that it's a late model receiver, if you have a sub (or LFE) output you could go line level from there to another amp to run the sub. Either way, you'll need an amp to push the sub.
To do it without another amp you would have to do something like get a high power L-pad to put in line with your mains so that you could bring them down to match the sub. That would be throwing away a lot of power, though.
If that's not the set or problem you've got then post up and we'll see where to go from there.
Ray
