SET on a xover type speaker does not work, I tried it, its a complete failure
Perhaps in your one instance, the complete opposite experience for me. My 3 way 5 driver speakers were built from the ground up to be very compatible with SET amplifiers. Same brand (Coincident) built by the same person (Israel Blume). Granted the crossover is a minimalist 1st order design (One tweeter capacitor). I'm referring to the Coincident Total Eclipse II, 94 db sensitivity and a 14 ohm impedance load.
It pairs exceedingly well with the Frankenstein MK II 300b SET mono blocks. 8 watts of pure music listening joy. It replaced my very good 100 watt mono blocks (6550 or KT 88). So once again it most definitely depends on what specific amplifier (All 300bs aren't created equally) and what specific multi driver speaker (They aren't all the same). This is my sublime experience the past 12 years.
Charles