You'll want a solid state amp that can go close to doubling it's wattage from 8ohms to 4ohms and then to 2ohms (which means good current delivery) if you want the best from the lower and upper bass, on these speakers. I would suggest a good big BJT (bi-polar) output amp like early big Krell's. And would stay away from Mosfet amps and tubes.
Quote from Stereophile tests.
"As taxing as this will be for an amplifier, there are combinations of 3.75 ohms and –37° phase angle and 2.3 ohms and +40° at 62Hz and 126Hz, respectively, which will also demand very high currents from the amplifier. As all music has considerable energy in this region, the Focal's owner will have to use an amplifier that can genuinely deliver high powers into 2 ohms if the speaker is to be allowed to sing as it should. Even then, as I found with the otherwise superb Simaudio W-7 amplifier, the high phase angle at infrasonic frequencies may well cause the amplifier to go into protection with a wideband pulse as it encounters what appears to be a short circuit on its output."