The Focal Sopra 2 is specified at 91 db/2.83 volts/1 meter, with a nominal impedance of 8 ohms and a minimum impedance of 3.1 ohms. Based in part on my experience some years ago with a high quality 18 watt tube amp and 90 db speakers having fairly benign impedance characteristics, listening to classical music having wide dynamic range in many cases, I would expect that for many (but certainly not all) users 20 high quality watts would be adequate. However a greater concern IMO would be the sonic synergy between the amp and the speaker.
Specifically, impedance curves I’ve seen at Stereophile for several Focal speakers, which in most cases are specified similarly to the Sopra 2 in terms of sensitivity and impedance characteristics, show impedances of 3 to 4 ohms in much of the bass region, and much higher impedances at higher frequencies.
The interaction of that kind of impedance variation with the relatively high output impedance of most single-ended tube amps will result in weaker bass and over-emphasized mids and highs, compared to many other tube amps and nearly all solid state amps. Although if your amp provides a 4 ohm tap the degree of that impedance interaction will be reduced somewhat, compared to an 8 ohm or higher impedance tap.
Good luck. Regards,
-- Al
