I used to go down to Criteria in Miami a lot during my college years. I knew Howie Albert and he'd throw reels onto the MCI machines and play things for me that were being recorded there (local stuff, not the BeeGees, Eric Clapton etc masters..which I'm sure even he didn't have the key to get the reels for). It was all great of course, but there wasn't an MCI "sound" as I ever experienced. In those days, I also heard a lot of 3m's, Ampex, and Studer machines.
Here's the way I see MCI in popularity and history....
Everything at Criteria from early on was MCI except for some Ampex Atr machines. Mostly because Mack (the owner) was friends with the MCI guys and MCI was located just down the street from Criteria. Both companies exploded into the stratosphere of popularity at about the same time. Firstly, because "Main Course" was recorded there in 75, as was Clapton's album with I Shot the Sheriff" (421 Ocean blvd or something) and a handful of other albums that just happened to hit really huge in the same one-year period....
.... AND, at that same moment, the independent studio craze was exploding between 73-76 (independent...not home studio) and people were taking out sba loans all over the place to open studios.
MCI was already creating equipment for Criteria and other studios on a more or less local Miami basis. AND, when it gets down to it, MCI was budget stuff. If you were looking to open an independent studio and you started shopping tape decks, you could get a 3M or Studer for $40-$45,000 dollars. Ah..but you could buy an MCI for $26,000. Big difference.
So this huge bunch of worldwide hits suddenly flies out of Criteria...Criteria is loaded with MCI....ten bazillion people are opening independent studios...they all open the monthly edition of R-E-P magazine and see those MCI ads..and viola....suddenly you have MCI machines in every other studio on the planet.
And MCI consoles became just as popular at the time because MCI could sort-of mass produce all this stuff and bring it to market at such..comparatively...great prices. Plus..from the console standpoint, MCI got togther with people like Michael Tapes etc, who all came up with a primitive..but workable vca automation system for the console faders. At a fraction of flying fader prices. Plus, in 76, MCI came up with a complete package for synchronizing their multitracks without one needing an electrical engineer to figure out how to wire it all together.
All in all, there was no way MCI wasn't going to dominate the world there for awhile. Which it did at full speed up until about when 3m released that first 32 track digital machine in 1977 and the world began to gradually shift to digital. Right after Sony bought MCI, it seemed MCI faded fast into the sunset.
Bottom line...I don't think there's a "sound" to MCI. MCI was inexpensive stuff to buy (on a comparative basis)... people like Clapton, BeeGees, and many others who loved Criteria just happened to record massive hits on the MCI's there because...well...because the MCI's happened to be there. Not because MCI had a certain "sound".
Even comparing a lot of Criteria/MCI stuff to Symzyck-engineered Eagles stuff, there's a big difference. Symzyck's techniques on the Eagles things are very "dark" imo. Sometimes almost murky with strange eq. Sometimes even a very prominent flat element rather than open/airy/3d. I attribute that to the technique, not the particular MCI recorder being used.
I'll say one thing. Criteria had some of the best sounding rooms I've ever been in. Even one of their tiniest control rooms was very very good sounding. Great acoustics and great monitoring in there.