You wrote :

"If the Qts is in the low 0.20's or lower, the driver belongs in a back horn. Performance in a quarter-wave pipe will be disappointing."

My latest thinking on these "back-loaded horn" designs is that almost all of them are really TL's for the lowest frquencies. If you look at the mouth area, multiply it by between 2 and 8 depending on room placement, you will find that the horn cut-off frequency is probably much higher then the designs claimed low end performance. So the enlcosure exhibits TL responses for two or three modes and then transitions to a horn like response. Compare Nelson Pass's Kleinhorn with the Fostex designs. There is nothing wrong with this approach and I am using it in my latest design.

Martin