From what I have read in the forums both the 975 and 976 are reported to sound better than the 990 although I would temper that with the "new shiny object" enthusiasm that pervades the audio world. I don't know how many owners of both the 990 and either a 975 or 976 have heard a big difference. The primary difference would be on the video side and the ability of the processors to decode the newer audio codecs internally.
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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