Well, hooking it up that way does work, and shouldn't overlead your receivers amps. But, you still have the problem Doug stated above. Plus you will have some crossover frequencies to muddy up the bass definition also.

I would think that your sub would have a speaker level in and out, thus crossing over the frequencies for you, you would select the crossover point on the subwoofer's amp. Make sure to read the manual to see what the main left and right out's are high passed at...that may help you select your crossover frequency better.