A while back Brian Wampler shared a bunch of mods to the classic MXR Distortion Plus circuit in Premiere Guitar magazine. It adds a tone control, a switchable soft clipping stage, a compression switch, and a bass boost switch.
This is a library of perfboard and single-sided PCB effect layouts for guitar and bass. I'm not an electrical engineer by any stretch of the imagination, just a DIY'er who likes drawing layouts. It is meant for the hobbyist (so commercial use of any of these layout is not allowed without permission) and as a way to give back to the online DIY community.