I have the 2805 and near the front of the manual it states not to drive speakers with below 4ohm, I'm sure the 3805 would be the same...
As far as the 600, the manual that comes with it gives you a good starting point. I have a huge room, and after calibration with an SPL meter, so far I have the following settings:
crossover knob: set to bypass as my receiver is managing the crossover of 80hz, all speakers set to small.
volume (gain): about the 8 o'clock position, the manual states to start about midway of 9 o'clock.
trim: set to flat as stated for a larger room.
phase: currently set to 0, you may have to experiment.
On my receiver I have the crossover set to 80hz and the dB level of the sub channel is on about -2dB. Have you calibrated with an SPL meter?
Welcome by the way and that is one bad system dude...