To me, the key here is not whether there may be some slight affects to the amplifer based on speaker reactances, but whether these small effects could even be actually audible, given wider range tolerances in many other parts of the sytem context.
As Audioholics points out, even normal changes in temp, humidity, and barometric pressure in the room makes more of a difference than some of these issues.
Additionally, practically no speaker made is going to have an "in-room" response of within 1db, so all of this brouhaha is virtually immaterial anyway. This output impedance/speaker reactance parameter is nowhere near to defining the outer envelope of system frequency response +/- tolerances in anyone's system. If we want to improve overall system frequency response, we can look elsewhere for the culprit.
If we want to really look for the differences in sound between tube amps and solid state amps, I think it is in the tone. The even-order harmonic distortion profile of certain tube amps(SET) have this profile when playing, not just when clipping. SS amps with wide-rangeing odd-order harmonic distortion profile have this profile when playing, and not just when clipping, too. Since odd-order harmonic distortions are not found in natural musical presentations, even very small amounts can be detected by the ear, and found to be an unconvincing reproduction. There may be other factors too, but I think that this is at the basis of it. Simply my opinion.
Before we jump on the bandwagon about this, is should be noted that push-pull tube amps are also rife with odd-order harmonic distortion, and so are multi-driver loudspeakers with crossovers. So it may not be(is not) just the amp that is causing this in most systems.
I find it interesting that some people want to focus in on a single parameter to back-up their prejudices about tube amps, when it is entirely possible, and even probable that inherent odd-order distortions in transistor amps/multi-driver speakers may be causing far worse sonic problems to real music listening, and they don't even know it. In fact, I find it rather humorous that as they chase a "spec" or "measurement" around, the sonic realism that is provided by a device that "specs worse", and therefore is "not acceptable", could actually be the very thing that they need to give them the convincing musical reproduction that they seek.
Sometimes I just have to wonder.
