A 'fully-balanced' circuit has a common-mode rejection ratio of pricisely zero, since all signal, common-mode or not, is simply amplified and passed along via the two signal paths. It then remains up to the following component to attempt to reject that amplified noise, if it has a differential amplifiee.
Thus, fully-balanced circuitry is subject to passing along any noise which might be picked up on all the cables. Then it hits the final component in the system, usually the power amp, where the differential amplifier at its input is left to deal with the sum total of the common mode noise in the signal path, (multiplied by all the gain in the system).
There are some real problems with first paragraph above! What is being ignored is that you can have a balanced circuit, and you can have a balanced differential circuit. Both are balanced. The latter is in common use; the former is rare!! The paragraph is written as if the former is simply the way it is and thus I regard it as misleading.
Here's how it really works. Most balanced circuits have a CMRR that is quite high- in our amps its up well over 100db. Any signal that is common to both inputs can't get amplified. The result is less noise.
Now with respect to the Bryston, on a number of them I have seen they have an RFI filter network on the non-inverting input, but nothing on the inverting input. The result is that noise can get amplified with this scenario as the CMRR gets reduced by this practice.
When you are building a differential circuit (which gets its name from the idea that it only amplifies what is different between its inputs) you want to get that Common Mode Rejection Ratio as high as possible! Low numbers usually refer to a design flaw or outright defect.
BTW transformers usually allow for fairly high numbers, so its possible to have single-ended circuitry internally, but if the input and output have transformers you can get really good CMRR numbers. This is how it was done in the old days in studio recording equipment.
However these days differential designs are well understood. A common misunderstanding about balanced differential operation is that the circuit will work better with a balanced input. The fact is that if the CMRR is high, the difference between using the balanced input as opposed to single-ended will not be significant. If you do see big differences between balanced vs single-ended operation, it points to a design problem and nothing else.
Now having said that a lot of our customers report that our amps sound better with balanced inputs and this is true. But its not so much because the balanced input is being used- instead its because balanced cables by and of themselves sound better than single-ended cables so long as they are driven properly.
This brings us to the balanced line standard which is poorly understood in high end audio. So here it is:
1) pin 1 will be ground
2) the source and the input both ignore ground and instead the signal occurs between pin 2 and pin 3.
3) the source will be able to drive 1000 to 2000 ohms with no loss in frequency response.
4) the cable will be a shielded cable with two conductors arranged in a twisted pair within the shield. The shield is ground only and does not carry signal current (see 2 above).
5) The outputs and the inputs of the electronics will operate with similar levels and gains, such that neither input (pin 2 and 3) are favored in any way.
If the equipment adheres to this standard there will be no hum and cable length will be nearly irrelevant. Also, the cost of the cable will be unimportant as one of the goals is to eliminate interconnect cable artifacts. The balanced standard is very effective in this regard. Oddly, it is this final aspect that one would think would be instantly embraced by audiophiles but I've seen a lot of pushback over the years on this point. Ironic, really.