If I understand correctly, the Vandersteen hookup puts a 6dB highpass filter into the loop, and then the sub is AFTER the loop, and receives a speaker-level signal. The sub’s internal lowpass filter is the inverse of that 6 dB highpass filter. Is the right?
If so, that makes sense to me.
Perhaps I misunderstood hammy1750 when he said he "would like to add a subwoofer into the amps loop function to take the load of the 80Hz and below off of the amp and DQ-10s and onto the sub." I interpreted that to mean the sub’s line-level input and line-level highpassed output would be connected to the loop’s output and input respectively, which would have placed them in the signal path before the volume control.
Edit: Looks like we were typing at the same time.