Tubes do not work harder driving 4Ω loads. The tubes drive the transformer primary impedance which remains essentially the same.Â
Transformers are happiest driving a matching impedance. When the impedance is mis-matched, so is the power. Tubes excell driving planars which have a mostly constant resistive impedance.
Roller coaster impedances like the B&W 800 Matrix are best served by transistor amps as they can supply the current necessary to generate the same power regardless of the load impedance. The B&W drop below 4Ω which can give some SS fits.
As far as sounding lean on 16Ω, the speakers are 16Ω at the very bottom, dropping to minimum in the upper bass and fairly constant ≈11Ω from 1k up. 16Ω could be just the ticket - depending on how the room affects the low end.
@tattooedtrackman Did you audition the AR amps before purchase?