Your concerns about driver QTS in dipole applications are understandable, but unnecessary. Anything with a qts below .8 (in some cases even 1) is considered qutie reasonable in closed box/dipole design. These drivers are not ideal for use in ported enclosures, but do fine in infinite baffle or dipole subwoofer configurations.

After all is said and done, qts has little to do with the sound quality of a driver (to a certain extent).

Having said htat, I don't feel the Alpha is a good option. It's unusual set of characteristics may give it a sweet sound for vocals and midrange, but it doesn't have the stuff to be a subwoofer. It also has very little xmax which is bad for a dipole setup, particularly if you are introducting low frequency EQ (which is generally what is done).

Possibly check out thise driver.
http://www.partsexpress.com/pe/showdetl.cfm?&User_ID=12695537&St=2137&St2=-81964001&St3=69117267&DS_ID=3&Product_ID=118185&DID=7

Adam