Oh come on Terry, you don't trust that better measurements equates to better sound? 
The Kepler is good but nothing is perfect. Part of the sickness of this industry is that you can never be satisfied with something. It goes for consumers and for most of us designing stuff. Once I get done with a project I'm scratching my head trying to figure out ways to improve it. It is a never ending cycle. 
Kevin,
I think you know that some measurements have meaning and others are without a lot of merit.
Amplifier THD and damping factor measurements are two that really don't have much meaning beyond a certain point. Julian Hirsch (and others) promoted standardized THD measurements years ago, as the industry used all kinds of bogus and semi-bogus numbers that told you next to nothing about the performance of the amplifiers. Way back when, in the early days of SS gear, an amp would have, for instance, "Music Power" specs. An amp would advertise 100 watts of Music Power, which might equate to 15 to 20 watts of usable, undistorted power with the rest being distortion and ultimately oscillation. "Music Power" was, IMO, the total power measured .05 nanoseconds before the caps blasted through the ceiling and the transformer became a grenade. Ok, that's perhaps a little exaggerated, but it was usually the last measurement that could be taken before the amp expired.
As time passed, THD became a standard measurement that established a data point that was reliable. Where the problem lies, is that it really is a composite figure, that tells you very little about the sound. Even order harmonics sound OK and odd order harmonics are nasty sounding, so any combination of harmonics could be totaled to equal the same THD figure. That was the point that the "underground" press harped on, that the subjective sound should be a factor in the equation.
Damping factor is another spec that has little relevance, again IMO.
Back in the early days of tubes, it wasn't uncommon to have a DF of below 20 or so. The Single Ended Triodes were even less generally. Damping Factor is, in part, a result of negative feedback, which triodes being perhaps the most linear device ever invented, didn't really need. With efficient speakers the Triodes could operate in their most linear region, which was at the low end of their output and as power increased, the levels of distortion also increased, although the 2nd Harmonic especially and even order harmonics generally, were the dominate forms of distortion.
Solid State (especially bipolar transistors) are essentially nonlinear by comparison and require copious amounts of negative feedback to correct the nonlinear behavior. The more feedback, the better the measurements of THD. Damping Factor figures also tend to rise, and it wasn't long before somebody got the idea that the numerical increase in DF specs was a good thing, allowing the amp to "control the woofer with an iron hand." The fact that an SE Triode, played within it's rated power, could achieve greater bass extension than SS at the same level, tended to be overlooked. The introduction of low efficiency speakers (AR 3a, as an example) pretty much eliminated the SET as a viable choice, as they ran out of juice just at the point that bass from these speakers needed even more to have any real "grunt."
You can check this out for yourself by using a power resistor in series between the amplifier and your speaker. The result is that the Damping Factor as seen at the speaker is severely reduced and you can often achieve an extra half octave of bass extension.
Three years ago I built a design that a friend (a Boeing Engineer) had come up with, for an event in Canada. It featured a second order series crossover that had an "optional" resistor. As I was going to an event that is nearly all tubes, and thinking that the option was for use with a SS amp, I left it out. The various posts resulting from the meet were "very" complimentary towards the speaker, and one post in particular mentioned that I had left a resistor out of the crossover. My friend saw that post and emailed me to find out what in the world I had done with his design. When I mentioned that I had left out the optional resistor for SS amplifiers, he was amazed. He stated that he had intended the "optional" resistor to pad down the tweeter a bit, if it was necessary. Anyway, we talked about it for some time and about a week later he called to tell me that he had run a number of simulations and found that the resistor didn't actually do anything to tame the treble, but it did extend the bass shelf about a half octave with his SS amp. The subjective results of this change of balance would actually sound as if the treble was slightly reduced.
Another rumor in the Myth of Damping Factor is the often cited "fact" that tubes (SETs in particular) have muddy bass, while SS with their high DF are able to produce solid, clean bass. This is partly true in many instances
if the impedance curve isn't fairly flat. If the curve "is" (hypothetically) flat, then the superiority of the SS amp's DF is rendered moot and, in fact, the lower DF SET amp may very well be able to produce more extended and powerful bass,
within it's specs.
This has actually turned out much longer than I had intended, but I can certainly see the advantage of an XBL2 driver, with it's controlled impedance curve. Mated to a tube amp, especially a SET (if the driver is efficient enough to offset the generally low output of most triods) it might end up being the speaker designer's delight.
Best Regards,
TerryO
