Why is it when testing the difference between 2 DAC/AMPs it only becomes apparent after you have listed to One for a while?

[MRC01]

Much harder when it's dac-amp combos
...
Those are good... But with two dac+amp combos or two different HP amps, a physical switch still is better than plugging and replugging cables.

Yep, then you use a physical switch, which can be transparent and virtually instantaneous when done right.

 

[stereo coffee]

Yep, then you use a physical switch, which can be transparent and virtually instantaneous when done right.

But better aiding transparency is to use a switch for audio paths with no physical contacts at all.

Noting concessions where using a LDR as a switch, that your application will need to tolerate a small Resistance On- typical is 60 ohms. Resistance off is typically above 20 mega ohms providing isolation to the adjacent input. In actual use however, you are unlikely to hear any audio from adjacent input, where resistance input to output is above 300k. https://lunainc.com/wp-content/uploads/2016/06/NSL-32SR3.pdf

The opportunity of course is to use concurrently the LDR both as a variable contact-less attenuator, and also as a contact-less switch

 

[solderdude]

I will let the panther judge your assertions. You have explained all this before.
LDR's are non linear semiconductors and add distortion. A potmeter is a linear element and adds far, far less distortion.

Proof for this is the Geshelli for instance as well as the Atom and all the other amps that show next to 0 distortion (reaching the limits of the AP555) which use cheap pots.