Which amp matches best my JM Lab Mezzo Utopia ?

The BAT solid state amps deliver lots of current, so that handles your load requirement, and are biased heavily into Class A. As such, they run hot, but I've owned BAT pre/power for the last seven years with no desire to make a change. Don't know them with the Utopias, but my Magneplanars are 4 ohms nominal, and there's never an issue of drive or control.

A friend of mine uses a LA audio 300B amp (mono amps 8 watt), it's a very fine combination. 3.8 Ohm isn't that low, if i remeber right they have a efficiency of 92 db, i think this is more important.

I read somewhere that it's not recommended to use SET amps with speakers that have impedance below 8 ohms.

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.