Sensitivity is one thing. Many/most older JBL, TAD, Tannoy speakers were high sensitive but they always needed some power from the amp to play under control. Those big drivers did not exactly have light weight cones and needed some damping from the amp to play clean. The impedance curve were also not exactly SET friendly (which expects 10 ohms+ at all frequencies for ideal playback). The Volti drivers are also inspired from one of these designs hence they have similar behaviours. With a low powered SET one can get good sound at relatively moderate volumes but once you decide to crank it up, the amps will quickly run out of gas and sound will lose its grace/compusure. I would suggest 15 watts minimum if one is going with SET.
On the other hand typical single driver speakers (Lowther, Fostex, Tangband) will go plenty loud with just 3 SET watts. It is a different ball game. The sound is probably purer but lacks the outright force and dynamics of the big drivers.
