ok, I need a quick sanity check. Amplifier input impedance is somewhat arbitrary for the Pass F4, but let's assume 47k, which is the default. Output impedance of my preamp is 100k, with coupling capacitor. My soon-to-be speakers have a f3 of 40 Hz and the driver I'm going to use in my future-to-be speakers has a fs of 48 Hz (it's a 15 cm driver), f3 undecided. I'm considering a 3uF capacitor with a small bypass, maybe 10nF. So, I'm getting 1/(pi*47k||100k*3uF), or fc = 1.66Hz. Opinions?

You left the 2 * pi out of your formula, but must have used in in your calculation. I would think this should be sufficient. On my recent coupling cap experiment in my preamp, I had used a 1uf on the output, given the 1M resistor to ground. Of course I forgot to account for the amp input impedance in parallel (10k), so the 1uf gave me ~14Hz... definite rolloff that I could hear. Went back to an 8uf cap as the stock one had been. No rolloff that I can detect by ear now, and RMAA looks much better.