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Old 7th October 2012
I own three different versions of the 501, I like them for some reason

Greenish-grey military sorta color front, rectangular push button - Valley People VCA - my favorite and most used. I upgraded to some fancier OP Amps (a real bitch of a modification) just to see what would happen, it did not make a drastic change, but it still sounds nice. There's also some kind of additional transformer in this one. I find that it has a fat and articulate midrange, and good balance. It's sort of a go-to on bass guitar for me.

Dark grey/blackish toggle switch - Allison Research VCA (early Valley People company name I think) - second most used. I put some NE5532 in here because it fried the OPA style ones I put in the other one from oscillation due to speed (or some such thing), would need additional modification to support those. I hear that the real benefit comes not just from the op amp swapping, but, by adding bypass capacitors to each one, and maybe some other things I'm not sure about too. This one seems darker and kind of bass heavy to me (on bass guitar, which I always try these on). I should probably try it again on various things, but so far I've found it a bit too heavy toned compared to the green one, at least for bass git. It works fine though and I probably still use it in some mixes on other things. The compression character is similar to the green one, but the EQ is different. No extra transformer in this one.

Blue, circular push button with the newer logo and different knobs - THAT 2150A VCA - probably least used to date. The compression character is slightly different on this one, hard to describe with adjectives. This one seems the most "middle of the road" of all of them, having fairly neutral sonics all around. I want to compare it to the DBX 160 but I guess it's sort of its own thing. They do not sound the same. Seems like it would work out for general uses, I liked it pretty well on a snare drum track I tried. I actually haven't used it much since I've had it, but I don't want to get rid of it.