Select means they have a lesser P/S in them, as evidenced by the specs, which makes them easier/cheaper to produce and achieve the THX certification. Ultra is now said to be for large rooms which makes sense as a larger room requires more power to attain reference levels.
THX has always just been a guide for those in HT to try to maintain a certain level of quality in the products. For me, I know Denons install good P/S and have very few issues across their entire line up so I do not look for THX with their avr's, other manufacturers I am not so sure of. The THX cert. means I can be assured, the avr can run my M80s to reference volumes in the designated room sizes as found on the THX web site.
