The only one i know that can answer this and experiment is
@stemiki as he powers his Dave out of a unregulated DC battery. If he uses tosink in and headphones his Dave can 'float' with 100% isolation.
Im also shure Dave can pick up RF from the mains.. even without earth connection.
I hear it easily if i place any (specially switchmode) PSU on the same mains outlet point as my Dave. Also together on the output of a mains conditioner.
I found this a time ago when i took my original HMS PSU brick's mains plug and put it on a different mains socket of my house which is wired to another switchboard group. Then again improvement when i did the same with my music server' PSU.
So the noise coming from my HMS PSU had less low impedance short line to my Dave PSU input.
What i heard was again less sharp treble and thus warmer sound with easier to follow and understand singer lyrics. The (bad) fatigue was gone after my optic link which improved things alot, but it was not the last word.