I had my original VSM-SE pair upgraded to MM (tube harness) several years ago and the difference was worthwhile. I haven't heard the MX. If you're buying used I'd recommend the MM. I've had my Merlins for five years (maybe more?) and have felt no urge to upgrade. They're amazing speakers and I always feel like I hear what goes in with very little alteration. They make all recordings enjoyable, but they disappear completely on high quality recordings. Highly recommended.
Phil
Phil