Well here is the scoop from THX.com on the Select, select 2, Ultra. I would guess judging from these the THX certification of select and select 2 means there is less power in the amp as it is only required to work in smaller rooms/listening environments.

http://thx.com/products/home/logos/index.html?f=h2

http://thx.com/products/home/speakers.html

http://thx.com/products/home/avreceivers.html

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70