I tried an experiment last night in which I hooked up a buffer to drive the subwoofer and it made a significant difference in the sound.
For the buffer I used one of these Schiit Loki tone control devices. Good stuff.http://www.schiit.com/products/loki
Normally it is used for EQ, but with all controls centered it makes a nice unity gain buffer. Its input impedance measures 50K. Not ideal but better than my 10K sub.
I split the PAS 3X outputs using Y adapters and sent half to the VTA70 and the other half to the Loki, which then went to the sub.
The Loki has a switch that bypasses the unit entirely using an internal relay. Perfect!
I turned the sub to mute for first test. So listening only to main speakers.
All PAS tone controls centered (bypassed)
With Loki active (sub is buffered), music sounds great.
With Loki bypassed (sub is not buffered), music sounds not so great. Lacking punch/definition is best I can describe it.
So essentially what I am hearing here is the difference between a 10K load and a 50K load on the PAS when tone controls are out of the circuit.
The X tone controls were in their center (defeated) positions. It was very easy to find the defeat positions while listening, as there is a very distinct position where the load has far less effect on the sound. This is the whole purpose of their design, of course. Just nice to see it in action.
If I rotate the bass knobs even just a little counterclockwise to move the control off of bypass, the difference between the 10K and 50K load is huge! In addition to the lack of punch mentioned above, there is also a significant loss of bass with the unbuffered 10K load.
(1) The PAS X tone controls work very well at bypassing the tone circuits, and in this position there is far less sensitivity to output load. i.e., they work exactly as expected.
(2) Even when the X tone controls are in bypass, the PAS output sound quality still degrades slightly when a 10K load is applied (subwoofer amp). It is not good enough to use under these conditions in my opinion.
(3) A 50K load sounds much better. I did not yet thoroughly compare the degree of further improvement going from 50K to the intended 500K, but from first impressions it is far less than the improvement moving from 10K to 50K.
So I suppose the take away here for future readers is this: Be very careful using a Y adapter to connect a subwoofer to a PAS amplifier *even* if you remove the tone controls. If the sub input impedance is very low you may loose fidelity from your main speakers.
EDIT: I can't tell from looking at the schematic why a low 10K Ohm load would reduce fidelity. Maybe someone else can explain this.