Digital gain vs analog gain

DAC = Digital to Analog Converter. If it has attenuation, it can be done mathematically before DA or post DA with digital attenuation or simply a pot, hopefully buffered.

As far as -10dB for 16 bit or -20dB for 24 bit being hinge points, that is utter nonsense.

In any system, matching gain structure is paramount and is best determined by inspection [listening or noise measurements].

Just because you don't understand something doesn't make it automatically "utter nonsense".

@ -6db you have given away 50% of the SNR. Digital or otherwise.

Then set amp such that your preamp can be run between 10-2 o'clock depending on desired volume ( therefor around 12) . This is usually the sweet spot on most variable pots used in a preamp. 

Data?

I have always been advised that one should set iTunes at full volume.

Compression is LOW-FI See ieLogical Lossy for a real life iTunes example

Lossy music on a high resolution system can be literally nauseating due to the constant atemporal image shift.