I see that there is a link for the manual:
http://www.partsexpress.com/pdf/300-808.pdfAccording to it, a Y-cable for the
L & R is needed to drive the amp to full output. It also explains why the
LFE would not be the way to go in a passive system:
Left/Right Inputs: RCA style jacks that will accept standard line level inputs from a pre-amp level source. They will
accept a stereo signal and internally combine it into mono. Both left and right input jacks must
be connected to the source in order to drive the amplifier to full output. The use of a “Y” cord is
suggested if a mono source is all that is available.
LFE Direct Input: RCA jack that will accept a signal from the mono LFE (Low Frequency Effects) output on a preamp
or receiver that is equipped with a dedicated sub bass output. This input bypasses the
amplifier’s own internal low pass filter circuitry, relying instead on the processed output
provided by the source equipment. This would be the recommended connection for most home
theatre surround sound receivers and pre-amps. We would also suggest the use of the LFE
input when an external line level electronic crossover or frequency dividing network is used.