Hi.
Nice to talk to another Korg owner. I have the MS20, and I’ve done a lot with it over the years.
I haven’t played the Mono/Poly, but I have played the Polysix and the Poly 61. I didn’t find either of them as varied and flexible as the MS series. I found the Polysix a bit characterless, though if I’d spent longer with it I may have got something more out of it. It certainly didn’t strike me as a stimulating instrument to play, though. The Poly61 seemed to be very limited, and the person who’s instrument I was playing felt limited by it and felt maybe she should have bought a Roland Juno 60 instead (this was the early 80s and that was the most likely alternative in the same price range).
Vince Clarke said the Mono/Poly didn’t seem to do very much. He said he fiddled with the knobs for ages without seeming to make any major changes. That seems to fit my experience with the other two Korgs.
I also seem to remember that the Mono/Poly wasn’t a true polysynth (hence the name?). I think it had four note polyphony but only one filter, but I could easily be wrong about that. It’d be something to check, anyway.
The bottom line is, don’t expect the early Korg polyphonics you’ve mentioned to have the sonic range of the MS series. Some of the later Korgs may have, though.
No doubt there are people here who can suggest a range of possible synths in your price range. I’d suggest anything by Roland, from experience.
These links may be helpful:
http://www.vintagesynth.org
http://jpmullan.com/stan
The bottom line, of course, is that any synth is a musical instrument, which means there’s an interplay between you and it. If it stimulates you to make music, it’s a good instrument. So the only real way of telling is to sit down and play.
As far as E-bay is concerned, be careful. There are probably some great bargains to be had on there, and some real opportunities to find something rare (I saw a Fairlight on there the other week). But if you don’t have the opportunity to try the synth you don’t know if it’s the right instrument for you unless you’ve played one before, and you really need to know for sure that it’s in good condition. Someone bought an ARP Solina on there recently, knowing it was what he wanted if it was in good condition. When asked what it was like when it arrived, though, he said it’d make a reasonable doorstop but not much else.
Maybe if you browse the site I’ve linked to, and then try to find somewhere where you can try a few different synths.