First Watt: current vs voltage

F1 and F2 are the two current source amps. I would recommend reading the 6moons reviews to get a good feel for each model. A First Watt dealer can also be of great help. Mark at Reno HiFi is a great guy and has an excellent return policy so you can see what works best in your system. He usually has most models in stock.

The First Watt amps are great. I have tried most of the F models, with the current source models (especially the F2J) and the F3 being my faves. But of course this is with my speakers and my listening preferences...

Roscoelli is right, 6 moons has extensive reviews on every First Watt model and compares/contrast each one.
The current issue has a complete review of the latest S1(SIT) First Watt amplifier.

The equation for Power is Voltage times Current. Audio amplifiers are power amplifiers. Speakers only care about the power delivered to their load. Amplifiers are rated in a maximum power delivered into a specific load. The amplifier only cares about the load that it needs to drive it's power into. The First Watt amps could care less what type of speaker they are connected to as long as the load is 4 or 8 ohms and that the load is fairly linear. Most of the First Watt amps are lower power between 5 and 25 watts. What you should be concearned with is speaker efficiency whether it is a single driver speaker or multiple driver speaker. The First Watt amps are the solid state equivalent of SET Tunes amps and indeed they have a similar sound. Single driver speakers happen for the most part to be very efficient. Nelson Pass happens to enjoy tinkering with single driver speakers in horn configurations but make no mistake they can drive any 8 or 4 ohm speaker with an efficiency greater than 92 dB at pretty good levels.

However, in man places I read that First Watt are current amps (in opposed to voltage amps) and thus they can only be used with single drivers speakers.

this makes no sense at all!! How did you conclude the 2nd part of your sentence from the first part??
Liguy has basically hit the nail on the head - all First Watt amps are power amplifiers. The First Watt amps seem to be low wattage, high(er) current amps made with a certain type of speaker load in mind (& that speaker load seems to be what I would call a benign load i.e. load impedance remains fairly flat 20Hz-20KHz & the phase shifts in that same freq range are minimal & not wild excursions from positive to negative & vice-versa).
I have a feeling, like Liguy, that the exclusive single-driver application being touted comes from the fact that single-driver speakers (which have no xover if there is no super tweeter) are benign loads. These amps should be able to drive any benign load speaker.