I finally figured out why the preamp was always necessary and how to remove it.

The problem was quite basic. I had always wanted to put a cap in parallel with the feedback of one amp or the other to act as a low pass anti-alias filter for the digital audio. I calculated the correct value as 2200 pF but wound up buying 0.022µF caps (that is, 22,000 pF). In working on Evil Simon, I figured this out and have been using 1000 pF caps ever since with great success.

Well, it turns out that if you configure the LM4871 for unity (it's actually 2x because of the push-pull design) gain with a 1000 pF cap in the feedback path, you can remove the pre-amp entirely and everything is just fine. The volume range is quite reasonable and at low volume, it sounds clean.

To do this with the existing boards, replace C25, R4 and R8 with 0Ω resistors, remove IC5 and C26 and populate C20 with 1000 pF. This effectively connects pin 3 of the volume control directly to the output of the balance control and moves the LPF feedback cap to the LM4871. If you're populating a new board, you can leave off C21, C22, R13 and R14 as they're not used for anything, but if they're already installed there's no harm in leaving them.

This will wind up being v1.4.