H500 and Lucas, Thanks for the info! H500, yes, the Rattm seems to have a string of problems. My only concern on the Haoyu is the temperature; I wonder if the idle current reduction wasn't working on mine, since it overheated at idle. Is idle current reduction automatic in the TB6600, or configured by the board? I couldn't find anything in the preliminary TB6600HQ datasheet, or the newer summary for the TB6600HG. I look forward to them being ~$2.
Lucas, yes, the Rattm has test points, and I found the Vref formula in the HQ datasheet, but I didn't know what Rnf was or if that older formula was still accurate. On mine at idle, I recall that Vref ranged from 0.5 to 2.8V while turning the amp pot from min to max. Thanks for the google tip on bulging caps. I traced pin 23 on each to its resistor, and both were indeed about 51k.
Here's the link to the Rattm I bought (they're on holiday like the rest of China now): New Product 4.5A TB6600 stepper motor driver board-in Motor Driver from Industry & Business on Aliexpress.com
The TB6064 kits look interesting but I want a presoldered board, and the higher amps of the 6600 helps a bit. I'll be needing a bunch of drives, which is why I'm reviewing the options more carefully. And there's one more: High Efficiency CNC Router Mill Lathe 1AXIS TB6600HG Stepping Motor Driver Board | eBay