I would first check your DVD manual to see if there is a configuration setting that downmixes audio tracks to 2.1, with .1 being for the subwoofer output.
If a music dvd or cd has no dedicated .1 track (as most normal cds do NOT), then there is no signal coming out of the subwoofer connection.
There may be a setting on your DVD player that does mix 2 channel audio discs to synthesize the .1 output.
If it doesn't then you would need to ensure that the DVD player is sending downmixed full range audio to the left & right outputs which are then connected to your Class Pre-amp.
From there, if it were me, I'd simply buy some rca splitters so that you can run a set of rca cables to both the sub L/R line level inputs and L/R inputs to your power amp.
Your speakers would get full range audio - so no 'bass management' - but it seems you run them this way now anyway.
I don't think there is any phase cancellation - if there were then your main speakers would see the same effect.
This is how I have my sub set up - get plenty of bass.
If a music dvd or cd has no dedicated .1 track (as most normal cds do NOT), then there is no signal coming out of the subwoofer connection.
There may be a setting on your DVD player that does mix 2 channel audio discs to synthesize the .1 output.
If it doesn't then you would need to ensure that the DVD player is sending downmixed full range audio to the left & right outputs which are then connected to your Class Pre-amp.
From there, if it were me, I'd simply buy some rca splitters so that you can run a set of rca cables to both the sub L/R line level inputs and L/R inputs to your power amp.
Your speakers would get full range audio - so no 'bass management' - but it seems you run them this way now anyway.
I don't think there is any phase cancellation - if there were then your main speakers would see the same effect.
This is how I have my sub set up - get plenty of bass.