Which amp matches best my JM Lab Mezzo Utopia ?

Blanket statement: you'll need ~50W class A or ~150W hi-bias class AB to get the Mezzo singing with appropriate dynamics. (They may be nice speakers but they consume lots of energy.)
So, given current pricing on quality hi-power tubes, I would recommend a SS.

Pass makes nice amps, but to use passive volume control you'll have to match their input impedance which is quite low if I remember correctly (check it out). OTOH, a low input impedance picks up less noise (mine is 10-12k Ohm across the board).

Thank you All!
Gregm, thank you - I have decided to go for a tube amplifier but will follow your advice to look for a 45-50 watt amplifier (may not be a pure class A amplifier though)!

When I purchased mine back in 2000, I listened them with a pair of Lamm parallel single-ended amps, some Thor monos, a Simaudio solid-state amp, and my own amp of the time, a McIntosh MC500. I definately preferred both the Sim and the Mac compared to the other two, mainly because of the bass response.

Since then, I've set up these speakers in five different living spaces, and the bass is the aspect of the Mezzos' performance that I'm least happy with . . . they can either get really boomy, or turn around (with position changes, or even different recordings) and sound thin.

The vintage Marantz amps I'm running now are by far the best I've ever heard with these speakers. They're P-P EL34 amps, and unlike most SE designs, maintain a pretty low output impedance. (Amps with higher output impedances are usually associated with the traditional "tubey" sound: relaxed highs, boomier bass).

And one thing I will say is that I have never once wished for my Mezzos to have boomier bass, in any room. Thus, I personally wouldn't be looking at single-ended amps as a good match for them. But as always, your mileage may differ . . .

Aleko, if you use a balanced amplifier with the single-ended inputs, the entire amplifier gets used.

Your speakers are tube-friendly but they need more power than most SETs offer, and the issue that you face with SETs is that the more power they have, the less bandwidth they will be capable of. You will definitely hear that on the JM Labs!

There are tube amps out there that have a lot in common with SETs (class A, zero loop feedback, all-triode) that will make the power that you need, and do it in a very classy way.

Atmasphere, I guess that you actually are the Atmasphere company?
Had read somewhere though that it is not efficient to use a balanced amplifier with an unbalanced source (as mine).
I am not targeting a balanced amplifier for this reason
as it is naturally more expensive due to the doubled circuitry.
Why pay for double circuitry and not concentrate the funds on a better quality single type circuitry.
A matter of strategy I suppose.
On the other hand I know that a balanced amplifier will provide more current at output which will be good for a hungry speaker like the Mezzo.