Edit: also to get a given voltage out of a microprocessor, you just use an analog-to-digital converter (ADC). Some have them built in, others are external. I like the AD9850 for hobby work, as it has a high resolution and is quite fast. If you need a large voltage range there are ADCs available for that, or you can amplify the signal from a lower voltage one.
WRT your issues with sensitivity to SMPS, it probably is audible noise. See if Screen Tunes[3] causes symptoms, it causes audible (but quiet) coil whine of varying frequency on most monitors I've tried. Your issues are unlikely to be directly caused by RF EM, but the side effects of the systems that generate such signals are often detectable by people.
[1] http://cds.linear.com/docs/en/datasheet/3574f.pdf [2] http://mmcircuit.com/understand-rcc-smps/ [3] https://thume.ca/screentunes/
I see. So set the current, let it draw the voltage it wants. Heh.
> Switchmode supplies can output any voltage, they're not limited to discrete steps.
Right - the issue I was getting at was the problem of getting an arbitrary representation of a desired voltage out of a microprocessor and into the real world. Digipots are the only way I'm aware of to do that (apart from PWM).
Thanks for the very interesting and highly informative wall of text :)
Open question, in case this piques your curiosity: I'm very interested what your comments might be on https://news.ycombinator.com/item?id=15005811, and any suggestions/ideas you might happen to have. I've long thought that what I describe in this post is noise-related, SMPS noise in particular, but I'm absolutely stumped on how to pinpoint/narrow things down any further.