You are correct about how I need to leave the ring disconnected.
It would be nice if I could use a smaller pre-amp, so I looked for some.
I had trouble finding a listing that discussed the electrical characteristics of the pre-amp, but I see that this other (stereo) board uses the PAM8403 chip, of which I found the datasheet. The attached table, from the datasheet, lists output voltages that look okay to me.
Maybe I’ll get both.
Does that sound right?