Which amp matches best my JM Lab Mezzo Utopia ?

If using a balanced amp on the unbalanced input : do both halves of the amp operate or not? If only one half of the balanced circuit and no intention to use a balanced source then buying a balanced amp is a loss of money. Anybody?

Sensitivity of the Mezzo is stated to be 92.5 dB. It is measured up to 2.8v. In 4 ohms this is not watt/meter. 2.8 v makes it watt/meter in 8 ohms. The sensitivity of the Mezzo up to watt/meter is somewhere 3dB less - 89.5 dB as the reviewers have measured it and stated it does not correspond to specs but without explaining the reason.

thats not vey demanding load i would get something soft in the top end for the jm labs

I am using Jadis JP200MC AND JA500 4 ohms. I think this is the best matching in my Mezzo Utopia system especially the mid range and bass is perfect. I don't think low watt or single end can control the bass. If you prefer low watt a smaller room may be accaptable.

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).