Answer, DJL.- I had a matched pair of MC012's from the Sound Room for a year, and have since sold them. They were replaced by an unmatched set of MC012's, and a pair of C-4's. I have used the 603's in a friend's studio for the better part of 2 days, so I can only claim moderate experience with them.
I find the MC012's to be very good overheads that border on great overheads. I do not like them on acoustic guitar. In 18 months, I never succeeded in getting a guitar sound out of the Oktavas that wasn't trashed by other mic selections, mainly SM82 and C414B-ULS. The MC012 is darker and more flattering than the other 2, and can be used by some for vocals, which was a little surprising.
The 603's, I find brighter than the Oktavas, better for guitar, and to my ear, not as cool as overheads. I think of them as a valid compromise between the needs of acoustic and drums. You'll notice my matched Oktavas got replaced by a cheap pair for $140 from GC, with the Russian manuals (no cedar box, alas). That pair (I lucked out, I think) turned out to be < 1dB in output apart across the audio spectrum, and have served me quite well.
The C-4's, I think, are quieter and flatter than either of the above. They sound more like a C414, just the facts, ma'am. I would not call them harsh or bright, I'd call them flat and accurate. They are not hyped or flattering, you know, like an AT4033 or Behringer ECM8000's without the hiss. (as omnis). I prefer the cardioid to the omni capsules, but I suspect that is mostly my room demons. They are, for my purposes, vastly superior to the Oktavas or, for that matter, the 603's, as instrument mics, and they are very good overheads for material where you want fast transient response, and snap. I haven't gotten to use them on piano yet, but I'm guessing they'll shine in that role. The best comparison I can make is to the SM81's, which I have also gotten to use, but don't own. To my ear, the C-4's sound more like an SM81 than anything I can think of. I hope that answers your question.-Richie