Just thought I would chime in one more time. I have about 150 hours in on my Manley Steelhead which is partnered with my Manley 300B amps-- replacing the receiver that was performing stand-in until the Steelhead arrived. After replacing a tube in the 300B that was apparently functioning but not at peak and inserting the Steelhead into the mix, I couldn't be happier with the sound.
With the Steelhead/300B combo, the Essence throws a huge wall-to-wall soundstage that starts slightly in front of the speakers and extends back through the rear wall of my room. In SET mode, they are absolutely stunning on small ensemble music or acoustic. It's pretty exciting to have Tori Amos, Ani DiFranco, and Van Morrison lifesize and in 3D in the room. Plus, if I want slightly better dynamics, I just switch the 300B's into PP mode. In PP, the 300B's have all the drive that the Essence will ever need in my room.
Upon final assessment, wihtout the option of switching between SET and PP, I would recommend at least 15 watts of SET or PP power to really experience all of the dynamics of which the Essence is capable. 24 watts of PP 300B power is more than sufficient. If you are not listening at moderately high to high listening levels on large-scale classical (e.g. Mahler) or on something like electronica (e.g. Massive Attack) then less watts would be just fine.
Enjoy,
PMB