Merlin and Joule Electra are probably the best combination assuming your VSM-M were designed for tubes. The Merlins ("tube" model) seem to optimized for OTLs, so I would focus, primarly, on that design for the power amp. The Joule OTL's are one of the best amps made, but are very expensive. Transcendent and Berning make very good OTLs that don't cost as much as the Joules and both are a great "match" with the VSM-M. I use the Joule Electra LA100 MkIII preamp and the Berning ZH270 OTL power amp -- it is a stunning combination!
But, the very best advise is to talk with Merlin's Bobby Palkovic; he knows his speakers and can guide you in your choice depending upon your budget.
But, the very best advise is to talk with Merlin's Bobby Palkovic; he knows his speakers and can guide you in your choice depending upon your budget.