Assuming you have a sub without it's own amplifier..........

Go to your local stereo repair or speaker shop and have them sell you a passive (Meaning no electronics!) low pass cross over. This will likely look like a coil of wire with two copper ends. Just wire it in line on the positive side of your speaker wiring. With your sub, you will want to have it start rolling off about 100 or 150 HZ, and it will likely only do about 6db per octave. This means it won't attenuate frequencies above the 100 or 150HZ very quickly, but for your temporary purposes, that doesn't matter much. Don't let them sell you fancier devices - they really aren't necessary. A basic "coil" should be a few bucks!

But watch your amp. If you have power meters, watch them carefully - you will be surprised how much more power low frequencies require to produce than higher. If you don't have power meters, make sure the amp doesn't overheat. If the resistance of the sub is low (Less than 4 OHMS.) the amp may not even run it. (It also depends on your main speakers.) Some amps have safety circuitry that will protect it in case of over loading. If the receiver doesn't work at all after plugging the sub in, that's probably what's happening.

Good luck!

__________________
Mike in Denver
'93 400E Now DEAD - Rear ended and totalled.
Replaced by '02 C32 AMG - FAST!