I bought one Watterott TMC2208 board and found that it was an easy drop-in replacement, compatible with my microstepping configuration, except the motor connectors needed flipping round else they ran backwards (if you're making a drop-in replacement for another product, and you have the option of making the motors move the same way as the other one, or the opposite way, why would you not just make them move the same way? the mind boggles).

Satisfied with this result, I tried to buy another one for the other axis but could not find another Watterott item on eBay, so ended up buying a no-name one, and it looks basically the same except something is weird about the microstepping modes, and I haven't been able to select a microstepping mode that is suitable, so I've given up on that for now. But at least I've got one quiet axis now, and if I come across a Watterott TMC2208 board, I'll know what to do with it :).