Read Herb Reichert's Stereophile review of the 86dB efficient Dynaudio Contour 20 with the SMALLEST $2199 PrimaLuna power amp. He stated "
The combo of Dynaudio Contour 20s and PrimaLuna ProLogue Premium played all the mountain ennui and fierce forward momentum that make classic bluegrass unique in the American songbook".
https://www.stereophile.com/content/dynaudio-contour-20-loudspeaker Or the 87dB KEF Q350 where he preferred it over amps with many times more power, and higher cost.
https://www.stereophile.com/content/kef-q350-loudspeaker Or this review of Wilson Sabrinas where the 36wpc $3199 DiaLogue Premium bested the $10,000 ARC REF75SE . The reviewer said "In spite of the low absolute power, the sound was incredibly captivating and dynamics were now seemingly more expansive than with either of the big transistor amps as well as the Ref 75."
http://www.hifi-advice.com/blog/review/loudspeaker-reviews/wilson-sabrina/ I've had a lot of Harbeth owners use PrimaLuna and can't think of any that were not happy off hand. If you like to play your music loud in a bigger room maybe an HP would be better, but for many people a 36 watt PrimaLuna will be fabulous.
I would never base any purchase decision of wattage. Wattage means nothing except volume. It has no effect on bass response or sound quality.