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.