A lot of nonsense happens when designers don't know about RF versus galvanic or audio grounding. Most of the solutions seen for one type, are horrid when considered by the other, and vice versa.
Single point star grounding, case in point. What an electromagnetic disaster. There is, however, an RF version which is general, covering both cases; the trouble is, few audio designers know it.
The system is the biggest problem in audio. You can't fix a badly wired system even with the best possible amplifier. Like I said before, you can't analyze a component in a vacuum, it's the system that matters!
In RF, you don't much care about ground loop currents, because they are rejected out-of-band, and isolation is easy (some coupling capacitors or a little isolation transformer). Differential signaling may also be used. You can make up for some sins this way.
In audio, the fault arises because ground currents are allowed to flow in signal conductors. Namely, signal cables are one pair and that's it: signal and "ground". Except it's not really "ground", or it shouldn't be: it's signal reference, negative. It's forced to serve both purposes, and the system is fragile as a consequence.
Oscilloscope probes suffer the same fate: they are both wideband, unbalanced, and sensitive to common mode (= ground loop) interference.
Commercial signaling systems avoid this by separating the signal reference and ground, and often using differential. Examples:
Shielded XLR, a direct analog example, differential
USB, a digital example, differential with poor CM range making it highly vulnerable, if not for its shield
ADSL, a hybrid example, differential, symbols are transmitted as phased AC with up to 32768 states
ADSL must be isolated? And Ethernet is transformer isolated (it uses differential signaling and relatively few states, e.g. PAM-3 for 100BASE-T), with the bonus that their signals are high frequency, so avoid low frequency noise problems doubly so.
XLR is sometimes transformer isolated, but this should only be needed in the absolute worst case, where ground loop voltage is so high that the receiver's CM range is exceeded (probably around 15V in most cases?).
The trick for "pro-sumer" audio is to use RCA jacks differentially. The shield must be RF grounded with capacitors. The shield can additionally be semi-grounded to the circuit or chassis through a resistance, but this should be nonlinear for protection only -- i.e., a large resistor to bleed down static, in parallel with a TVS to shunt fault currents from cross-wired circuits. The differential voltage is then sensed with a diffamp, and you don't have to worry about ground loop currents on board at all. Mind to include an RF filter there, to clean up remaining trash, especially if you're using slower or older or bipolar circuits that love to rectify RF.
Such a method won't solve CM currents already on the cables, or between other equipment in the system, of course.
Tim