Updated June 15, 2007: Check out the Ibanez Tube Screamer History page. It's clear(er) now from checking out this site that it is in fact the TS-808 that would be the ideal pedal in this series. Analogman offers custom mods to the TS-9 that give you exactly what you need. I want one of these!!
I have been interested in getting this pedal for a long time. A vintage model is probably prohibitively expensive but I have also read the the TS-9 reissues do not use the same circuitry as the originals. (Another Hammett reference for those who read my last post where he is quoted in Guitar World Jan. 2006) I've read that it is much less expensive to get the reissued model and have it modified to get the vintage sound then it is to buy an original. So what difference is the modified circuitry making to the reissues that is missing in the first place?
Everybody's favorite, Wikipedia says, "...The classic Tube Screamer sound includes a "mid-hump," which means that the circuit accentuates freqencies [sic] between the bass and treble ranges (mid-frequencies). Many guitarists prefer this sort of equalization, as it helps to keep their sound from getting lost in the overall mix of the band."
A couple of articles for electronics folk delve into the "secrets" of the Tube Screamer and Tube Screamer modifications. Rather then modifying a stock reissue model, it may be simpler to obtain that classic tone by going to another source: Maxon. A company who apparently built Tube Screamers for Ibanez from 1971 -2002. The Maxon OD-9 looks like it is just what I am after, and the OD-9+Pro looks interesting as well with quite a number of options for, "...effectively function[ing] as a 4-voiced monster, offering everything from the classic, bluesy tones of the original 808 to the searing saturated chunk demanded by today’s extreme metal music."