Hello Chuck,
I have a Powers Fotopanel (which works OK), a Honeywell - Sparco thermostatic mixing valve, and most recently a Haas Intellifacet that Ralph recommends. It is superb. I wish I had bought it first. I use the Intellifacet for precision control and the Sparco for less demanding control when I need two water temperatures at the same time. The Haas has a small transformer/power supply that plugs into a regular wall outlet. A GFI is recommended, but that is a good idea in a wet darkroom anyway. The Haas is worth every penny, IMHO. It has fixed temperature detents on the dial, but they can be adjusted if other temperatures are required, as can the linearity of the dial.
Cheers,
Clarence