7.
With the stock decoding algorithms on 5.1 sources, you get a bit more immersion from the side + rear surrounds. THX-EX encoded source will put sounds arround, beside, and behind you. Your Lexicon will also enhance pans from the front to rear speakers. My upgrade from 5.1 to 7.1 with a Lexicon DC-1 was worthwhile.
As far as the sub:
Home theater bass requirements for reference level LFE playback are ludicrous. Few commercial sub-woofers will not reach 115dB @ 20Hz @ 10% distortion.
With the stock decoding algorithms on 5.1 sources, you get a bit more immersion from the side + rear surrounds. THX-EX encoded source will put sounds arround, beside, and behind you. Your Lexicon will also enhance pans from the front to rear speakers. My upgrade from 5.1 to 7.1 with a Lexicon DC-1 was worthwhile.
As far as the sub:
Home theater bass requirements for reference level LFE playback are ludicrous. Few commercial sub-woofers will not reach 115dB @ 20Hz @ 10% distortion.