Thanks Guys, I'll check out Supergen. Two positive posts on the place! Would still like to know the differences between the three models.
I did contact Supergen via Email about the different models. Here's the responce;
They have 4 colors. Red, black, gray and yellow.
73531 yellow and 73538 red are both 1600/2000. We sell refurbished on both. Red has DC output, yellow does not.
73536 yellow, 73537 gray and 73540 black all are 1700/2000 and all have DC outputs. Yellow is our standard model. Gray is made for Northern Tool.
Black is our newest model with USB output available.