Total power available to a channel is somewhat dependent on the power supply. The 7075 has a power supply capable of providing at least 525 watts into all 7 channels at the same time (7X75). Each channel is not capable of providing much more than its fair share since you essentially have seven 75 watt amplifiers in the case. Being that most of the amps built by ATI for Outlaw are pretty conservatively rated you might expect to get a few more watts out of each channel at full power. Now for the reality part...the rears, sides and center will never approach the full power of each channel on a continuous basis. The use you have for the 7075 is perfect for what you are doing. Powering the surrounds and letting your mains get more power from another source gives you a little more headroom on the power supply and may let it last longer.
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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