I don't know about your receiver, but typically a sub like the LFM1 will do a lot better job of reproducing an 80hz signal than the average speaker. Not because the speakers can't handle it, but because the receiver does not have the output to drive them properly without something else suffering. The LFM1 has its own amp, and therefore will leave the receiver with a bit of headroom for the rest of the speakers in your system.

Also, usually on a receiver you can tell it you have no sub and send the line level output through the LFM1 and use its crossover and continue on to your front channel speakers from there.