Well I bought a "Cartoon Classics" boxed A500. Sadly it was not the A500 Plus I wanted (and used to own plus 2MB chip), didn't have any of the original software and has a duff modulator (rectified - I just bought one from ebid). The machine is completely non-yellowed however.
The 501 looks like it might be on it's way out but I'll add solder and lacquer it to get a few more months/years.
Really don't know why I bought it apart from price (£10, or about $17). I use my good condition 600 for games, 1200 is the gem in the collection and stored away, and the 500 will be "throw around" - probably... hopefully not throw out.
At this point I'm thinking I should have used the space for a C64...