This discussion on double bass has went on way to long. It should not be used if you have a capable sub. A sub should be able to produce better bass than large towers. Double bass can lead to cancellation issues, phase problem and in general delivery poor bass performance compared to setting all speaker to small. Here are Audysseys recommendations:http://www.audyssey.com/blog/2009/05/small-vs-large/
Subs can play all the way down to 20 Hz where as the so called large towers can't. Double bass leads to boomy and bloated bass. Audyssey as 8x the filter resolution for the sub compared to the other speaker and should give better results.
The use of variable xo is not necessarily good also. I see people xo the center at 40 Hz. this leaves to much bass in that channel and can affect dialogue. For HT systems, most people should xo their speakers 60-100 Hz. Since the key to getting good bass in a room is location of the speaker, it make sense to move the sub around to find the best spot. One cannot move around the other speakers.
Also the bass resonance in the room will linger with multiple speakers set to large and there is no way to sync them all at the sweet spot;/s.
A full range speaker cover 20 Hz-20 kHz or need 2 twelve in. woofer with around 200 watts for amplification. Or, the speaker needs a 15 in. woofer. Most of us don't use speakers that fit this definition, so use bass management with all speakers set to small!!!!! You can't find a respected source recommending double bass.Edited by derrickdj1 - 4/9/13 at 2:37am