Weseixas, actually with many tube amps when you increase the load impedance the output power goes *up* not down.
In the case of transformer coupled amps, this is because the OPT is more efficient. In the case of OTLs (and especially if its a *smaller* OTL), this is because the entire output section is running more efficiently- more power is dissipated in the load and less in the output section.
Bass extension has nothing to do with output impedance. Bass 'control' might, but since damping plays such a minor role here, what really is important is distortion. That's what gets you muddy bass! So if the amp is undistorted its more likely to be perceived as 'control'. Just FWIW.
Anyway, if the speaker was designed on tube amps, its unlikely that a transistor amp will exhibit more 'control'. My impression is that transistor amps in this situation exhibit 'punch' but not a lot of definition or subtlety. I often find myself trying to turn them up to get the same impact I expect out of the tube (in this case, OTL) amp. I guess my point is that so much depends on the interface between the amp and speaker that its very hard to make generalities.
