What are called "ground loops" are usually just AC current in the ground lines, originating in capacitive coupling of the power transformer primary winding to the signal ground of the device.
In the simplest system, you would have a source (your DAC) then the Quickie, then a power amp. Both the source and the power amp have power transformers, so there will be some AC current flowing between them, on the ground lines of the interconnects and through the ground connections of all three devices. The voltage drop across the non-zero resistance of this ground path appears to the various input stages as if it were signal, so you hear hum and its harmonics. Since the coupling is capacitive, the harmonics come through more strongly than the fundamental, giving it something of a buzz character.
If more devices are connected, there are more sources of this ground hum/buzz and it gets harder to isolate the culprit - for example, unless the DAC is connected optically, the input to the DAC will likely be another source.
Think about how many pieces are involved in that ground path - each RCA jack and plug has a pyysical contact surface and a solder connection, plus there is the ground wire in the interconnect and the ground wires in the device (including Quickie). A lot of opportunities for a tiny bit of resistance to creep in!
Balanced lines greatly reduce this problem, which is why they are used in large, complex systems and those with long lines, such as recording studios and landline telephones.
