Matching to speakers is critical with SET's. An easy load can offset lower sensitivity in some cases. Low sensitivity and impedence picks are not a good match.
I have a pair of Gallo Nucleus, the original 1 ball speakers with the capacitive tweeter. There is no crossover. They are extremely easy to drive and 90dB/1w/1m. 300B great, 2a3 actually pretty good, the 45's did well on female vocals, small ensemble jazz and chamber music. My room is a bit largish, 16X40 with a cathedral ceiling 14'. IMO the key is no weird crossover issues and/or impedance peaks.
My buddy has Thiel CS3's, 89dB/1w/1m and none of the above amps, (nor my 40w 6L6GC PSE amps) sounded good...pinched frequency range and thin bleached timbre. Thiel crossovers are fairly complex and they were not a good match with any of my tubes amps, although almost, (to me), listenable with Rogue Audio Magnum M150's. He is a sand guy, but wanted to hear the Thiels with tubes. He is now looking to replace the Thiels.
As in all things, YMMV....
Cheers,
Geary