@kijanki instrumentation amplifiers use laser trimmed resistors internally and/or external precision resistors as well as significant open loop bandwidth to achieve high CMRR and fixed gain levels. If you mean the differential pair / long tailed pair on the input, they also need somewhat matched resistors and in op-amps they are trimmed to provide higher CM range. <<$1.00 op-amps have 100db+ common mode rejection, but not in practical circuit which would be about 70db with 0.01% resistors. Low cost practical instrumentation op-amps with high CM range readily achieve 90db CMRR. Your amps may have a few precision resistors, and/or they are calibrated.
The main point of differential connections in audio is to eliminate the noise from having ground references which are different at the receiver and the transmitter whatever they may be and how they are caused. In a home audio system, that is going to be predominantly through noise induced through the AC, with the capacitance of the power supply, even EMI caps completing the circuit.
With a cartridge, you only have a ground connection on one end. Equal currents induced in the same direction on each wire (common mode) "induce" the same voltage and cancel each other out (it is a loop), at least at practical audio frequencies. Differential noise, will of course look just like a signal. The cartridge is already "isolated".
The main point of differential connections in audio is to eliminate the noise from having ground references which are different at the receiver and the transmitter whatever they may be and how they are caused. In a home audio system, that is going to be predominantly through noise induced through the AC, with the capacitance of the power supply, even EMI caps completing the circuit.
With a cartridge, you only have a ground connection on one end. Equal currents induced in the same direction on each wire (common mode) "induce" the same voltage and cancel each other out (it is a loop), at least at practical audio frequencies. Differential noise, will of course look just like a signal. The cartridge is already "isolated".