Runoff Groove's Supreaux Deux is a JFET overdrive that emulates the Supro 16T. One of their best circuits -- it really has the spongy feel of a small Class A amp. The volume knob cleanup is "shelf-like," similar to a lot of vintage fuzzes. The volume output is pretty insane. I also like that the tone control is faithful to the amp -- and not crazy at all; it's a second order filter with a high cutoff frequency.
I built mine with a charge pump to run on 18V internally. It's a tiny circuit so of course I stuck it in a 1590A. I did leave off the bass boost switch, which adds a lot of saturation. In the end I preferred the clarity of leaving it off, and it would have been a very tight squeeze getting it into the enclosure.
There are many layouts for this circuit around the forums. My perfboard layout, with the on-board charge pump, can be downloaded from my web site.