I do not think 20 wpc is enough for low efficiency speakers in a small room, just not loud enough for some material/occasions. Some peaks will be out of gas.
Low efficiency speakers, I would want a 30-45 wpc tube amp, and that's too much heat for a small room, so, rather than solid state amp, I would go for efficient speakers, a 10" woofer helps.
Let's not forget the Iron Law, for Bass, you gotta move a lot of air. I prefer larger woofer to smaller 'long throw' and ports, and using corners to get more out of a 6-1/5" (or even smaller), or use a single non-directional self-powered sub like I did with the Wharfedale's 6.5" vented.
You want to remove the low frequencies from your main low powered amp, so neither the amp nor the speakers try to produce low bass.
I am enjoying my 10 wpc tube amp, just enough with the 90db efficiency AR-2ax. More power would be nice on rare occasions, or more efficient speakers would be smartest, but what might sound as good as these AR-2ax's.