Well, depending on who you talk to, or what it is about old TUBE sound that YOU like, your mileage can vary.
I like the first issued C-20. It sounds more like a Mac MC-30 amp; midrange magic galore first and foremost. Almost immediately they redesigned it and made it more, er, neutral, almost boring (in comparison to the original design, NOT other preamps).
Regardless what one might read on other web-sites, no way in heck does a C-20 sound anything like an MX-110; two more different sounding preamps I have never heard.
At any rate, any C-20 is going to give you neat lifelike sound. The earliest version will sound more euphoric and luscious whereas the later revisions worry more about adding a bit of extra top and and bass.
It of course totally depends on what speakers you are using and what amps as to the sound you are going to get. I miss my C-20 every day.
None of the later McIntosh tube preamps ever sounded like this one. It's a keeper...
Looks like there is an early one on eBay right now. I have no idea who the seller is...
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=3057249603&category=14974