No doubt the RME choice of power supply is not critical ...
I switched from the cheap wallwart to a 12v Li battery and the improvement was nothing less than astonishing. Whether the wallwart's feedback back into the mains played a role I cannot say. My wallwart is plugged into my mains 24x7 as I use it to charge the Li battery when not listening, so there has to be more than just that.
Perhaps the wallwart causes less issues when it does not have a load on it? I'll pull it out and see if I hear a difference.
I did try a friend's linear power supply and it doesn't match the battery. I think I left the wallwart plugged in while using the linear power supply but I cannot say for sure.
Long story short, the battery beats all comers.