Looking at the speaker and its specs you'll need a pretty powerful amp, although a lot depends on your room. As far as I can make out (Focal is pretty precious about what specs they are willing to publish), this speaker is four ohms in the bass and 8 ohms in the mids and highs. They rate it at 8 ohms but dual woofers are involved, so the 8 ohms is probably due to the fact that the cabinet is ported, because those woofers are probably wired in parallel. I expect that is also why the impedance dip is around 3 ohms. This speaker is intended to be driven by a solid state amp.
So you'll be using the 4 ohm tap on the amplifier, although you should try the 8 ohms tap too. With tube amplifiers the efficiency spec is easier to use than the sensitivity spec, because tube amplifiers do not double power as impedance is halved. Focal claims 91dB; converting to efficiency you get 89dB. In an average room in the US you'll need 200 watts to play any music at volume comfortably. If its a more lively room 100 watts will do alright.
You'll want to keep the amps close to the speakers if you can (monoblocks are good for this) and keep your speaker cables short, probably no more than 6 feet.
If you really want to see what tubes can do, a more efficient speaker and one of higher efficiency would allow the amp to better strut its stuff. Tube amplifier power is expensive to get right; this is why there are more efficient loudspeakers.
