Make Headphone DSP with just 1 biquad

Subtracting the left and right channels could be argued to
separate the center channel from the left/right channel and the
DSP therefore gets applied to panned sounds only. This halves the
cost of the filtering and eliminates the bass boost issue that bs2b
compensates for with a highboost filter.

I also adjusted the delay-to-level spatial filter to try to get slightly
more consistent perception of panning across the frequency spectrum.