Indeed. But it is also worth noting that a balanced input (unless item is fully balanced from in to out) comes at a cost, be it hiss (active) or THD (tformer). If an unbalanced op has extremely low Z, terminated to an unbalanced IP, it may well give better noise performance. Self has written about this, and anyone who has spent time designing with a noise analyser will know for themselves. GML mic preamps have a balanced IP but unbalanced OP for this reason.
Interestingly, although I am a big fan of balancing tonearms, most MC carts have incredibly low Z. This means that, in measurements, I have noticed virtually no difference in noise of the same cart to SUT whether unbalanced or balanced. Were there significant EMI / RFI sources in my room I suppose it could be a different story.
Hiss is relative to the signal levels and gain structure, natch. I think there are two things going on here. Differential circuits by nature improves SNR by 3 dB (signals double, uncorrelated noise RSS's, so signal goes up 6 dB and noise only 3 dB), so comparing an active single-ended circuit to an active differential circuit, the differential (balanced, I realize not always the same thing) circuit will have better SNR. However, comparing an active differential circuit to a single-ended or quasi-balanced circuit where the higher signal level is not used, the single-ended circuit will win.
I suspect what
@Speedskater is referencing is the benefit of common-mode rejection that differential circuits provide, that can provide 20 dB or more of common-mode noise rejection and potentially reduce hum by breaking a ground loop. All that is very implementation-dependent (at the circuit level).
I no longer have a TT set up. When I did, I found the same thing as you -- MC cartridges, while may pick up motor hum in a poor design, by and large were less sensitive to noise than the high-Z MM cartridge input stages. Hiss at the speaker varied, however, since the MC carts tended to have lower output and thus required an additional gain stage. Usually any added noise was buried in the noise floor of the recording itself so not noticeable.
IME/IMO - Don