It should work, and would provide a means to drive L&R via either the preamp or the receiver, depending on which was selected. Reading the specifications in the "Manual" PDF, however, indicates an audio mode THD of 0.01%, probably a little more than an AVA amp. To my mind, this seems like it would be increasing the THD by maybe double and decrease the sound quality for 2-channel stereo.
I was thinking about HT bypass and came across this device on the Inday site.
http://www.inday.com/av4x/av4x.htm
The L/R preout from the receiver and the output from the two-channel preamp would be connected to the Inday's inputs and the Inday's outputs would be connected to the two-channel amplifer's inputs. The two-channel amplifier is connected to the fron left and right speakers. The listener would merely need to select the correct input (the HT receiver's preout or the two-channel preamp) for HT or two-channel.
Would this device work?