> I've always been surprised that there aren't more preamps and recorders which feature a single global gain control that adjusts the gain of all channels simultaneously, keeping them balanced relative to each other, in addition to individual channel trims.
Like it or not, you're asking the manufacturers to choose among (a) the convenience of ganged controls, (b) the accuracy of their gain settings and interchannel balance, and (c) an affordable selling price. You can have one, or at most two of those things--but you definitely can't have all three at once. And since the consumer or "prosumer" equipment market is so heavily determined by price, you're basically asking NOT to have accuracy when you look for a control scheme like the one you describe.
It is very difficult to build ganged analog level controls that maintain accurate balance between channels across a wide range of settings. By "difficult" I really mean expensive and time-consuming (which again means expensive). The manufacturer has to keep wiring potentiometers into the circuit, testing them, and rejecting them until the desired degree of matching is found. For any given parts and assembly/testing budget, all other things being equal, you will get better results with separate, unlinked gain controls in each channel. So I wish I could persuade you to stop wanting what you have described, or else accept a version of it that uses digital gain controls, which don't have these limitations.
Among analog equipment, I don't know whether you're familiar with the Grace Lunatec V3, but the gain control arrangement of its preamp section seems good. The basic gain for each channel is set in 5 dB increments with stepped controls that use discrete resistors (i.e. each setting of the knob selects a different discrete resistor); the basic gain of each channel can thus be set to within a small fraction of a dB of the nominal value, and the gain of the two channels will match almost exactly at each given setting. Then there's a smaller, second knob for each channel that allows continuous adjustment within ±5 dB around that basic setting per channel.
And back to the original question in the thread: For coincident or near-coincident recording with directional microphones, even matched pairs of condenser microphones from a good manufacturer aren't necessarily matched as well as one might ideally want. If you have a stereo recording that you've made using X/Y or ORTF or the like, try bringing it into your favorite editing software, selecting your favorite 30-second passage, and listening to it a few times. Then reduce the gain in one channel by, say, 3/4 dB and listen again. It's different. Perhaps it will be equally pleasing or maybe even more so--but it won't sound the same, because our brains are extraordinarily sensitive to differences between what our left and right ears are hearing. Try the same thing by restoring the balance to what it was originally, but this time try adding 1.5 dB at 100 Hz in just one channel and not the other, then listen again.
The fact is, no manufacturer will guarantee that a matched pair of their microphones will match within such close tolerances that the difference between them (i.e. switching which microphone is used to record which channel) could never be heard in precisely the kinds of stereo recording that most of us do. When you order a matched pair from some manufacturers, you may get two microphones that are within that limit, or you may not, although your odds are definitely improved by preferring matched pairs (particularly of capsules, since that's where the main variations occur).
Finally, since there is no standard definition for "matched", an unselected pair of brand "X" capsules could very well match better than a selected, matched pair of brand "Y" capsules if company "Y" has less consistency in their production to begin with--and there really are major differences among manufacturers in that respect, and even among different microphone types from the same manufacturer. It's not just a matter of competency. Not every manufacturer strives equally hard for consistency in manufacture, since most market segments place less value on it than we do. Customers who think in terms of pairs of microphones don't dominate the marketplace.
--best regards