With the smaller box volume I would think that the 976 will feel warmer because the case is so much closer to the electronics. At the same time the electronics may be more or less efficient thus generating more or less heat. Hard to tell but the 990 power draw is 50 watts according to the rear plate whereas the 976 says 40 watts. You have the right setup with fans moving air/heat away from the equipment so I would not expect any issues.
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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