One of the Best High efficient Loudspeaker from Audio Note is AN-E SPe HE model.
The crossover is simple, essentially first order 6db. In the time domain both drive-units are connected with the same positive acoustic polarity
You will not hear good sound from low efficiency Loudspeakers and the most important key to have good sound is going for High Efficiency (above 94db) Loudspeakers. for best result the Loudspeaker should have flat Impedance curve above 6 ohm.
Most Low Power Single Ended Triodes could drive AN-E SPe HE easily.
I will Demo AN-E SPe HE in my Showroom as soon as Possible.