If they really don't make balanced inputs for such low level signals, they should. My current power amp (Benchmark AHB2) has CMRR=80dB (equivalent to 0.01% gain matching) Good Instrumentation amp, like one in my Rowland amp, have 90dB CMRR (and it is just plain line-in). As a result of both you will get very poor CMRR order of 30dB only. Phono stage amplifies 60Hz by about 6 times vs 1kHz signal.
Matching them by hand is not practical for production, not to mention changes over time (aging). As for matching RIAA curve - it is very difficult. Matching resistors is unnecessary if cross-referencing of both "legs" is used (like in the input stage of instrumentation amp), otherwise it is almost impossible. The other is that even harmonics produced by both "legs" cancel, but some people looking for "warm" sound might not see it as particular advantage. But the OP asked "why" we don't have (more) balanced phono circuits, and I think it's mainly the profit One advantage of true balanced is to remove reference to GND in order to get rid of wire to shield capacitance, but the same can be achieved with input transformer. I have two fully balanced phono stages neither of these ever drove me crazy with hum and noise problems that I read about all the time in relation to SE phono stages. (Your argument about RIAA equalization does not hold as much water as does the argument about tube or transistor gain matching, because it is much easier to exactly match capacitor and resistor values, or to tweak the values for good balance, than to match the gain components.) To all of that I say, so what? CMR is reduced from some perfect value to some lesser value, but you still do get CMR. Kijanki, To the degree that signal handling by the two phases of a balanced circuit is not exactly symmetrical, you are correct in saying that will reduce Common Mode Rejection, and it does for sure in tube-based balanced circuits, because exact matching of vacuum tubes is not possible or stable even if it's momentarily possible.
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