So I turned off the minimum extrusion temperature so that I can send over a print without heating up the hotend first. It didn't seem like the extruder wanted to move at the start of the print at all until I tried spreadcycle mode. 

The peak was 53.3C and the S6609 cools down fast, so it makes sense why it would be somewhat cold by the time I was able to open up the DP2 and put my finger on them. The cooling rate could be increased due to the heatsink. This test print was performed with a motor current of 800mA.

I tried 0.01 linear advance and it sounded horrible while I got these messages:

[16:54] Reading this, I enabled Square Wave Stepping in Marlin and now spreadcycle is much quieter. Stealthcop still doesn't work and linear advance still sounds like a screaming skittle, but I no longer hear each individual line of infill. It's more like a slight rattle (likely slack in the metal BMG). 

[17:07] So it looked like at 800mA (actually 795 according to M122) was going to converge to 55C. Changed the current to 600mA (581) and temperature dropped to 47C. I might keep a thermistor in the box just so that I can test stepper driver temperatures, because I wasn't expecting an 8C drop from a 200mA power cut.