Zlew, it's one of the Atari ST's timers used to repeatedly change the volume in a channel of the YM between minimum and whatever it's set to play at. Because the timer isn't 100% the same speed as the notes being played on the YM, it creates a pulse width effect. Thus "SIDSound"
Yup, maxYMiser it was. I would have composed on real hardware if I hadn't been an idiot and got a STe instead of an ST.
Also, thank you, MiDoRi. There's a DSP support chip on the STe that lets you filter the output of the YM, so you can get even more SID-like sounds. Though, that's probably out of scope of the AYM category.