Which amp matches best my JM Lab Mezzo Utopia ?

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.

Aleko, wherever you read that tidbit, its not actually true. Also FWIW if the amplifier is balanced-differential you do not have 'doubled circuitry'. Differential circuits have the advantage of lower noise and lower distortion as opposed to their single-ended counterparts. This can be a big deal depending on the complexity of the amplifier. In single-ended circuits the distortion of one stage is compounded by the distortion of the following stage. Obviously a fully differential amplifier will be lower distortion.

Lower distortion is usually audible in 2 ways: more transparent (detailed) and more relaxed (regardless of bandwidth).

The Mezzo is a tube-friendly speaker, but does need some power to make it go. You will be a lot better off with 50-100 watts than you will be with 7-15 as a result. The speaker will literally wake up!