May be a bit tricky, a lot (ie most) pro recordings do a lot of processing on the way in, essentially what goes 'into' the tape machine or computer is already pretty 'mixed' by the standards of what most people consider mixing. There are a LOT of benefits to doing it this way, but you have to be pretty solid in your audio 'decisions' otherwise you could make some mistakes you can't go back on!
This does mean that if you get the splits for a 'pro' track you will find there is not that much too mix! There is normally still a fair bit of work to be done, and it IS a brilliant learning method but 'pro' mixing is often a lot more subtle and technical than 'home' mixing, where you are often sorting out a lot of things then and there, and still making some strong creative decisions.
I have splits of some pretty killer tracks, my collegue managed to find loads of stuff but I only nabbed 3 Marvin Gaye tracks, grapevine, nothing like and whats going on. Then 4 nirvana splits, superstition and long train running.
As I say, none of these splits sound that off from the released tracks when put into a multitrack editior, with a bit of level fiddling. But reverbs are not often there (they would have been on desk sends/aux's for the mixdown and not recorded back into the tape machine) and its DEFINITELY a good way of practicing mastering/master buss eq and compression!
Also, with some of the bigger sessions, for example the Marvin Gaye stuff, there was actually a lot more tracks than you 'get'. The version I have is just the master tape, ie the vocals are already comped (with an 'alternate take'), the backing vox is all one track, brass section all one (or two maybe) track etc. They had recorded it to several tapes/machines then come mixdown put the sections onto one 'master' reel, so you don't get to hear the complete splits.
Still very cool to play about with. I'm kinda a little off about spreading this stuff, as I don't think it was meant to be leaked, maybe a cheeky assistant or other member of staff crept in and grabbed it from the master reels at some point. But that said, for people wanting to work in music and learn from the greats its very useful, so I feel less bad about 'donating' it as a learning aid rather than hosting it for the whole music-enthusiast public to pick apart. If there's the demand I'll host a couple on rapidshare, but as I say, you'll be surprised how little 'mixing' there is to do!
At very least its interesting to hear all the parts of the song laid out. And how each thing should sound in isolation and silly little things like how much 'spill' is too much, some of the tracks are surprisingly 'imperfect' by modern digital standards (where you would probably cut the silences in vocal takes etc.) but still sound great! You also hear how three mics on a drum kit can sound Kick-Ass (all I mentioned apart from nirvana, which is a more modern set up, though Its hard to tell if some of the drum tracks were 'bounces' of more than one drum mic to the master reel. If they are its a subtle blend, not an obvious one).
Some of the individual instruments don't sound all too inspiring on there own but sound amazing in the mix, and THATS a BIG thing to study even if you don't actually end up doing any MIXING as such!
Its also great for learning compression, if you try to compress most of the signals on the splits you'll hear it just messes everything up, takes it far from the original etc. It's really good for learning compression discipline and knowing when to stop/what it should sound like to work in a mix. Its all starting to sound very zen
man, how do I always end up writing such long posts?!?