Reference 3a Royal Master Control loudspeaker
The match between amplifier and speaker for single-ended operation is critical, which is why John Atkinson suggested I let Wavelength's Gordon Rankin lead when it came to choosing a speaker to partner his special-edition Cardinal amplifiers. If you're not talking roughly 92dB sensitivity and a fairly benign load, say no lower than 6 ohms, you're just not talking single-ended. That precluded running the Cardinals on the big Avalon Ascents with their lowish 86dB sensitivity (in spite of the relatively benign impedance curve).
After more discussion it became evident that the best speakers to use were the very ones Gordon employed in his own reference systemthe Swiss-made Reference 3A Royal Master Control loudspeakers.