As an aside, you'll notice that I used to consider the Professional Series offerings to be "tube friendly" until experience showed that their performance on SET amps was marginal in some cases. So maybe the 2226's work well for some people but not for others, as one might expect from its moderately high Zmax. It isn't terribly high, but isn't low either. It probably works well on some amplifier configurations and not on others.

Also, some very interesting articles on the technical issues surrounding interfaces between electrodynamic speakers and triodes have been written by Steve Bench and by Norman Koren. I suggest not only the articles directed by these links, but also the others at each authors' sites.

About how two drivers will interact, at low frequencies, they will combine and even act as acoustic mirrors to one another. It's like having them up against a room boundary in the way they reinforce one another. But as frequency rises, there comes a point where the two begin to form nulls in the room. The pattern looks like dual slit diffraction experiments, with hot spots and dead spots.