Thanks for your help, kchapdaily.
I did a bit of testing, and I changed my mind.
I compared the OBH-11 against an O2 set to 1x/2.5x. Using a full-scale 100 Hz sine wave (2.26V RMS) for input, I measured the voltage output of the O2, max volume, no load. The results were, respectively, 2.1V and 5.5V RMS. With measurement errors, etc, the results seemed to me to make sense for unity gain and 2.5x. Then I measured in the same way the OBH-11, and the result was 9.7V RMS (with massive clipping). I don't really know if I was interpreting these results correctly, but it seemed to me that the OBH-11's gain could not be anywhere near the 23x suggested by values of R6 and R5. It looked like it was closer to 4x, and this was confirmed by listening (at much lower volumes, of course).
So I decided I didn't really understand how this amp works, and I'm not going to spoil a perfectly good, working amp -- at least until I have a better idea of what I'm doing.
But thanks again for your help.