Wow, the advice about using 3 subwoofers sounds VERY expensive....and the interaction between the three drivers would probably be a huge mess!!!
Anyway, it sounds like the general advice coming in is correct: You should set your system up as if you have LARGE FRONT SPEAKERS AND NO SUBWOOFER. Then run the FRONT L/R outputs to the subwoofer as well as the front channel amplifier inputs. (Hopefully you have two sets of outputs on your preamp for the FRONT channels. Otherwise, you could buy two Y adapters to split the outputs.) Now you can adjust the subwoofer settings (crossover, output level, phase) so that the front speakers and subwoofer mix together correctly.
This method makes sense because the ".1" channel (subwoofer output for "5.1" system) will contain frequencies that are too high to put into your subwoofer. (They would be much more accurately handled by your front speakers!) With my system, I found that my Thiel CS2.3 front speakers were good enough in the low bass that I got the best results from my subwoofer (Definitive Technology SuperCube I) by turning the crossover as low as it would go (40 Hz.) It might have sounded even better if the subwoofer crossover could have been set lower since the Thiels should respond down to about 35 Hz. I found the best position for the subwoofer to be in-between the front speakers with all three speakers at the same distance from the listening point. This gives the most natural response. Have fun.............Bufus
Anyway, it sounds like the general advice coming in is correct: You should set your system up as if you have LARGE FRONT SPEAKERS AND NO SUBWOOFER. Then run the FRONT L/R outputs to the subwoofer as well as the front channel amplifier inputs. (Hopefully you have two sets of outputs on your preamp for the FRONT channels. Otherwise, you could buy two Y adapters to split the outputs.) Now you can adjust the subwoofer settings (crossover, output level, phase) so that the front speakers and subwoofer mix together correctly.
This method makes sense because the ".1" channel (subwoofer output for "5.1" system) will contain frequencies that are too high to put into your subwoofer. (They would be much more accurately handled by your front speakers!) With my system, I found that my Thiel CS2.3 front speakers were good enough in the low bass that I got the best results from my subwoofer (Definitive Technology SuperCube I) by turning the crossover as low as it would go (40 Hz.) It might have sounded even better if the subwoofer crossover could have been set lower since the Thiels should respond down to about 35 Hz. I found the best position for the subwoofer to be in-between the front speakers with all three speakers at the same distance from the listening point. This gives the most natural response. Have fun.............Bufus