I read online that the sub is intended to cover frequencies below 150 hz so it is probably already doing all it can at this point. The LCRs are all 3.5 inch mid-range speakers so they won't be much use down low. I would think the built in subs will add a lot of oomph to your system. The sub in the corner is getting all the boundary gain effect it can use now so if it sounds good I would leave it until you can get the new speakers into the system and check out some good LFE movies.
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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