I'm a little surprised that a dealer would recommend a 300B (7 wpc?) with the Kensingtons by referring to Jeff Day's review. Jeff has the Westminsters which I believe are at least 6 dB higher efficiency than the Kensingtons (99 dB vs 93 dB?).
I don't own Tannoys but I'm interested in them and have auditioned everything from the Kensingtons up to the new Royals (the latter for fun, they are way beyond my budget). I suspect that smaller amps can still sound very pleasing - if played at moderate levels in smaller rooms. But if owners listen to symphonies, big band jazz, or rock, more power would be a benefit.