Even if your speaker has a high efficiency, I do not recommend small power SET amplifier to drive two 15 inch drivers.I can't imagine why.
My speakers are 98 dB with dual woofers. My preamp has a fair amount of output and in a pinch, I can use it to drive my speakers and it controls the 15" woofer just fine. An SET would do just as well; the big problem is 'will an SET have enough power?' and if a more powerful one that does more than 7 watts then the answer is 'most of the time'.
I prefer a bit more power; much depends on the distortion that the amp makes- if low in overall distortion (in particular the higher ordered harmonics) then it will be relaxed and you will very naturally use the extra power.