What amp should be used with merlin vsm's?

Pedrillo, I'd give a listen to the Quicksilver V4 tube amps, which put out 120 watts per chanel. They are very liquid, have very good 3-dimensionality, and a convincing presence. They are about $4000 used, but worth it.

waltcertain

EAR 890 tube - 70 wts P/P. Can be had used for under $3,500.00.

BAT VK60 or VK75 with V-Cap TFTF coupling caps. I'm using a recapped VK75SE with VSM-MXs. Transparent, neutral, authoritative with Merlins, particularly when biwired. Hardly a typical configuration, though.

I've used a bunch of different amps with my VSMs over the years and most have worked out very well.
My personal favorites include:

Cary 805 (might be over your stated budget, but really excellent)
Cary 300B monos (magic with vocal music)
Pancor Dyna re-issues (Power + pre should be under $1K used - if you can find 'em. Sound is really good, but these eat tubes and then get noisy. Even so, this is the best value I've come across).

These also worked well, but were less striking to me:

ARC VT 130SE
KRELL KSA 50s
Odyssey Monos
Pathos Classic One Integrated
Original Quicksilver monos

Good luck,

Marty

FWIW we have a lot of customers running Merlins. I doubt that its coincidence...