Back in the day there was an Acoustat dealer that I visited on several occasions. He drove the speakers with the VAC Renaissance 70/70 amplifier (Push-pull 300b, 65-70 per channel) with truly excellent results. Granted these amplifiers were built with very stout power supplies .
I did not realize that their sensitivity was that low (81db) so I understand The recommendation for higher power solid-state amplifier. But, I can attest to the fact that they were marvelous sounding driven by the VAC amplifiers. The speaker probably has a friendly impedance load.
Charles