Well, the good news is, the amp works great.
The bad news is that once I was driving the iMac speakers with a usefully powerful sound source, it became apparent that the speakers are blown. Damn.
But it’s definitely not the amp – that part works great, on speakers or headphones, from multiple sound sources. Good sound quality, power levels as expected… if only the speakers weren’t shot.
Ah well, it was still a fun build.