Tubes, to damp or not to damp?

Not a fan of any of these devices. Very simple solution if your tubes are microphonic then replace the tubes. If all your tubes are microphonic then replace the component. In all but a few poorly designed components microphonics is rarely an intrinsic problem. If you like the sound of dampened tubes then more power to you.

I think it is situational to the gear and the tubes.  And some manufacturers seem to design their gear around the tubes with dampers, so it is part of the "voice", as it were (ARC for one).  But these damping devices aren't expensive, and it isn't hard to try them out and hear what you hear.  Herbie's will let you try and return if you aren't grooving on it, which have taken them up on more than once.  My experience with their dampers has never been positive with the squishy clear silicone cylinders strung on the teflon parted-ring.  But I have used the FKM O-Rings strung on the metal parted-ring style with success.  Different horses for different courses could well come into play, but that is my experience.

I would estimate that dampers have worked in ~40% of cases when I tried them over the years, but it all gets down to the particular gear, the particular set of tubes, my system's voice at the time and my preferences - - and, of course, the particular dampers.  Tweeks take work to tune right.  Hard to categorically state that dampers are all good or all bad, IME/IMO.

Tube dampers screw up the thermal heat transfer of the tube and should not be used!