I would expect a more flexible ability to decode anything Directv throws at your system with the 975. I am not sure that the processor in your TV is as capable of fully decoding a 5.1 signal coming for the HD channels of DTV. Bear in mind that although DTV says all their HD channels are 5.1 audio out that is a capability statement. Actual audio output will depend on what you are watching. There are many variations of "Dolby Digital" and the capabilities of each are quite varied. Maybe your TV can decode all the various permutations but I doubt it can do it as well as the 975. The 975 is designed to be able to deal with all the permutations.
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Music system
Model 990/7500/Magnepan 1.6 QRs/Technics SL1200 MK2/Aperion S-12 Subwoofer/OWA3/Sony NS75H DVD
APC H15 Power Conditioner
TV System
Large Advent Loudspeakers/ Polk center/Monoprice surrounds/Panasonic Viera 42 inch/Onkyo HT-RC260/Sony BDP S590/Directv
Home Theater System
Onkyo PR-SC886/Outlaw 7125 Klipsch RF-82 L/R,RC-62 center, RB-35 SR/SL, BENQ HT1075, Outlaw LFM1-EX/OPPO BDP-83/Directv
Harmony ONE
Blue Jeans and Monoprice interconnects
APC H15 Power Conditioner