With the benign load requirements of the VSM, an OTL amp is probably the best option overall. Joule, Atmas-phere, Berning, Transcendent.
So far as transformer coupled tube amps go, there have been numerous ones that would probably work quite well with this speaker. Bobby showed the CAT a few years back at one of the shows. Greater dynamics is my understanding but it is hard to imagine too much out there that is going to beat the Joule, Atmas, or Berning.
You should hear the VSM/Berning with David Berning's tube BAM, it adds a special flavor to this combo, sweeter and much better harmonic resolution, the music is a bit closer to being real. You couldn't want for too much more. I can't give a SS recommendation because I haven't heard one other than the SIM with the VSM and while sounding good it didn't provide the magic. I've heard a few other SS amps that were totally forgettable with the Merlins.