Nicely done! Who'd have thought you could get four gang reverse audio taper pots? I don't see any DC bias on the positive input of the first opamp however. Wouldn't you want to tie that 1k resistor on the left directly to the transistor emitter rather than through a 1uF cap and maybe bias its base a bit above half supply? The two 6.8nF caps could go to earth/ground rather than Vref, I suspect you started with a Sallen-Key high pass where those two caps were resistors and so would have provided the positive input bias when tied to Vref. I should have run my circuit at unity gain. I thought there was going to be an advantage to running at higher gain but that turned out to be wrong so I backed it down to almost unity gain, no idea why I stopped at "almost"! Someone who feels two poles is enough but wants better battery life could use a single opamp and your input buffer, or use a dual opamp with the other half as the input buffer, assuming someone sells two gang reverse audio taper pots. Normal audio taper pots could be used but they would rotate the wrong way.
