I'll say that I have settled on Q = ~.55 on my VTF-15h.
I really liked how it dug REALLY deep with the Q set at or near .7.....the bias toward low frequencies didn't bother me at all for music, but kick drums (think rush, led zeppelin, etc.) lacked definition and tightness. Having it shaded just past the .5 setting preserves the "depth" and extension, but really tightens up the sub for music. After messing with room placement, I found that there was relatively little difference in definition between .3 and ~.5 for my room....there was a HUGE difference between .5 and .7. It's a very effective control and I'm glad Doc included it in the design. Suddenly, any sub that doesn't have it seems lacking.
Oh, and the darn thing sounds fantastic
on music. Goes great with my Arx A5 towers, which are themselves critically damped designs (with a Qts of roughly .5). They ate my old eD A7s-450 for lunch and spat it back out; it just couldn't match the tightness and control of my A5 towers. The HSU keeps up so well, it's difficult for me to tell where one ends and the other begins..... color me impressed.