The stomp boxes are simple state variable filters. I have one in the form of the Madbean Bloviator II that was small enough to stuff in the same box as a compressor that I built. It really does a trick with acoustic guitars. The rack units have a proprietary VLSI chip in them with considerably more going on inside. That's the description in post #39. I've got one that I use with my stereo tube gear, and it's just for fun. "Enhancement" is a general term, but it works. The amount of enhancement needed may elude some. It doesn't sound good with everything, but with most things it does.