Theoretically, How to Reduce Gain without Minimal Impact to Sound?

Well, if you can't hear the difference yourself, does it matter?  I mean, your own experiments are going to be better than whatever the prevailing or mythical wisdom is? But often DACs have linear volume controls that avoid the worst of the number altering behavior of the early digital volume controls.

Having said that, you could put a passive volume control before your Freya, and maybe even make a custom L-pad out of high quality resistors. 

I have Benchmark DAC and power amp.  Benchmark recommends setting DAC's output high and amp's input low.  That way more signal amplification is done in quiet environment (DAC).  In addition it will be more immune to ambient electrical noise induced in the ICs. You have to experiment because other things might be affected.  In my DAC changing output level changes output impedance.  

Should I cap the volume level at my (digital) source to something less than max/100, like, say, 65, while leaving Gain to High? Or should I leave the digital volume maxed, but change Gain setting down to Medium or Low?

Since you have an option of gain settings (High, medium and low) try the lower setting positions. Low nay be the best choice for your scenario.

Charles

Traditional resistive gain pots are a negative in the signal path.  Most of these pots have a contact that the max setting that shorts across the pot so that it is totally out of the circuit.  You benefit greatly from having these pots maxed out.

Your digital source gain probably doesn't use that and I've found digital gain in the applications I've used it, did not degrade sound.

The volume attenuators on your monoblocks could be anything.  Do they have detents like you describe on your pre?  if so, it is likely you have a resistive ladder, which if made with quality resistors is minimally disruptive.

And as Charles said, looks like you have a low, mid, high setting.  that is also likely resistors and not degrading to the signal.  So try that.

Bottom line, sliding resistive potentiometers are the one thing you want to keep out of your signal path.

Jerry