I am finding this is a very interesting thread! I want to add my experience of trying to use a non- gated digital device in front of my Fender Super Champ(not set to a very high gain. My mind goes wild imagining all of the potential of being able to utilize flexible DSP algorithms in front of my tube amps whether the digital devices were hardware or software based. Imagine the tweak-ability especially for a tweak head such as myself! What I found was that All of the devices and their DA converters did raise the noise floor somewhat. Of course the higher the amps gain the greater the noise. Now these devices are all very high quality and expensive as if price really matters:-)
It’s just the nature of the beast I have found. I like to think of it as a tube guitar amp input is an acid test for ANY digital device because of the often times higher gain settings that most of us like.
Basically I prefer to use time based effects in an effects loop ir in a Twp amp wet/dry rig so at line level signals the hiss issue is not an issue.
I do like my ProCo in front of my amp though/-)
My 2 cents.