Hi Lewm . . . so for the subject of balanced inputs for phono preamps. First, its important to understand exactly what type of noise a balanced input is capable of rejecting. The overwhelming majority of cartridges and tonearm/turntable cartridge wiring treat the cartridge winding as a balanced source -- this alone is wholly sufficient to avoid hum pickup from ground-currents flowing between the turntable and preamp, provided no mistakes are made. Suceptibility to RF interference is determined by the design of the input stage itself. That leaves magnetic hum pickup as the main type of noise that we're designing the balanced input to reject.
The effectiveness of the hum-rejection in a balanced-input system is directly related to how closely matched the positive and negative conductors are impedance-matched to ground - that is, if the + and - conductors on the input have different impedances, the magnetic field will cause different amounts of interference in each one, which in turn will manifest itself as signal voltage. This is why (IMO all decent) balanced-signal cables use twisted-pair or star-quad configurations - the tight twisting keeps the impedance very much the same between the conductors, for good hum rejection.
But in a tonearm, the wiring is very rarely twisted-pair . . . they're usually just stuck side-by-side through the tonearm tube. And this isn't necessarily a bad thing, because the side-effect of the twisting is an increase in cable capacitance, which is exactly what we DON'T want for our MM cartridge. And if you want to minimise capacitance in the leads between the tonearm and the preamplifier, then you're also probably looking at simple coaxial cable types instead of shielded twisted-pair. Then there's the matter that having carefully-balanced output impedances is probably only very rarely considered in the design of the phono cartridge . . . because the vast majority of phono preamps over the years have unbalanced inputs.
So in any case when we're designing a phono preamplifier, we CANNOT assume that the impedances coming from the + and - leads of the cartridge will be well-balanced, impedance-wise. So if we want magnetic hum rejection, we need to build a balanced input that is fairly insensitive to these imbalances. And the way to do this is to keep the differential-mode (cartridge loading impedance) as low as possible, and keep the common-mode impedance as high as possible, as it's the ratio of these two that determine the effect of the source imbalance.
So for a traditional, say 5-ohm LOMC cartridge driving a 50-ohm transformer input, this is pretty easy to obtain, because the transformer will have a common-mode impedance in the tens of megohms, say 50 Meg. The ratio between the common-mode and differential-mode input impedances is thus 1,000,000,000 . . . and since the cartridge source impedance is so low, the maximum impedance imbalance will be a fraction of an ohm anyway. So hum rejection can be reasonably effective, regardless of the type of wiring used.
If we're to do this with an all-active input, we would still have the same 50-ohm loading resistor, but to effectively manage i.e. input-bias currents and offset, the input impedance of each side to ground would be probably at the highest maybe 470K, making the common-mode input impedance 235K, and our impedance ratio 4700. Not as good as the transformer, but still worth it.
But when we go to an MM cartridge, the source impedance is usually something like 1.5K and rises with frequency, so the impedance imbalance of the cable is then more likely to be a handful of ohms, and also rising with frequency. Per our earlier discussions, the differential-mode input impedance needs to be about 100K. And assuming JFET inputs, the very highest you can probably get away with for common-mode (without having the offset go through the roof, or cap-coupling) is 2 Meg resistors . . . making the common-mode impedance 1 Meg, and our impedance ratio is at 10.
So the endgame: the higher output impedance of an MM cartridge will make the impedance mismatches in the wiring more apparant, and at the same time makes the necessary design criteria in the phono preamp more suceptable to these imbalances. And the most effective way to reduce the imbalances in the wiring (twisted-pair construction) raises the capacitance, which is exactly what we DON'T want for our MM cartridge.
So then there's implementation - for MM cartridges, we can't use transformers, which leaves us with active realizations, which have a couple of major disadvantages over unbalanced inputs. The first is noise . . . you usually end up with twice as many uncorrelated noise sources, and can only make up for it by the fact that each side will see half of the impedance, giving a minimum 3dB noise penalty. The second is that many input stage designs don't work as well in the presence of significant common-mode voltage (which if we're trying to reject it, means it exists), and with high common-mode impedances, some sort of protection diodes, series resistors, etc. will probably be necessary to keep the input stage from getting fried when a ground wire gets disconnected and suddently there's 30V of common-mode voltage.
While I won't pretend that my conclusions on the matter are definitive, all of the above makes me think that for an MM cartridge, active balanced inputs are unlikely to deliver enough hum rejection to be worth the complications.
