Hello! So.. yes, this is purely from memory, and so I wouldn't put an enormous amount of stock in what I say
However, I do feel that I have an ability to remember sound signatures accurately from the past, so let's see..
I would say they are quite different in most ways actually
I loved the Mason V3 that I was loaned for that review and would still love to own it, especially the turquoise coloured one that I reviewed!
It had a very well done low end, among the best I've heard in an all-BA IEM.
Now, when I say "the best", I don't mean "the most powerful".
Although I'm a bit of a bass-head, there was something about the Mason V3 low end that I just found really engaging.
It was very tight, fast and accurate, relatively neutral on the mid-bass, but with enough sub-bass extension to dive deep when a track required it.
Both IEMs have a big holographic soundstage, outstanding separation, imaging and very detailed and revealing upper-mids and treble.
MEST is more W-shaped in signature, whereas I felt the Mason V3 was more of a classic neutral natural kind of tuning.
I feel the MEST is more bright with more sparkle in the treble, and the Mason V3 is a bit more calm but with a lovely shimmer.
MEST low end is a more contemporary style; hard hitting, impactful, rumbly.
I'd say they're probably kind of equal with regards to musicality; I feel the MEST is a remarkably accomplished IEM, but the one small thing I'd like to tweak about it if I could would be to introduce a touch more warm and richness into the mids, to add a bit of note thickness and weight to the timbre.
Despite that, it still sounds extremely musical and engaging, but that would be my chosen addition.
Overall, the MEST is pretty full-on. As I said, a quite contemporary tuning that's dynamic, exciting and engaging,
Conversely, the Mason V3 is like a modern Bentley or Aston Martin, sleek elegance, power and poise. Unruffled, even when racing at speed.
It's highly engaging without ever being overwhelming or fatiguing.
Well, these are just some thoughts, and I hope you and anyone else interested would find them helpful