Furman, Panamax, Tripplite and APC all good stuff. Assuming the system isn't using some enhancements (in the processor) to the audio such as treble boost or some sort of equalizer, the brightness is likely due to your room conditions. You can hang stuff on the walls to reduce the frequency bounce somewhat. The fact that the room is large and opens into another room is part of the problem. I doubt your speakers are making the sound brighter and although many people say they can hear big differences in amp sound I don't think that is your issue either.
Model 990/7500/Magnepan 1.6 QRs/Technics SL1200 MK2/Aperion S-12 Subwoofer/OWA3/Sony NS75H DVD
Large Advent Loudspeakers/Panasonic Viera 42 inch/Onkyo HT-RC260/Sony BDP S590
Home Theater System
Onkyo PR-SC886/7125/Klipsch RF-82 L/R,RC-62 center, RB-35 SR/SL, BENQ HT1075, Outlaw LFM1-EX/OPPO BDP-83/Directv
Blue Jeans and Monoprice interconnects
APC H15 Power Conditioners