It varies wildly depending on the speaker and the amp. Certain brands are best with huge amounts of power and are not just low efficiency low impedance as your rightly pointed out.
That said, with speakers of even modest efficiency, amp quality is often more important that power. I run Wilson-Benesch Discovery 3Zeros with a pair of 55w mono blocks. I have run with more power (200w) into 8ohms via excellent SS amps and do not feel they outperform my better tube based monos.
I just did an extensive test between my Canor tube monos and the AGD Gran Vivace. AGDs delivered slightly better bass response. Canors sound slightly more natural. But these are small differences and most people would walk away saying both were great and a great match for the speakers.