I would definitely try them with the subs engaged, with and without your old sub in the system. Since the placement is limited by the design I would place the older sub in your main seat and do the sub crawl with everything in the mix to see if you have a better place to put the old sub. With two subs bouncing off the walls and a third in the corner you may have some areas where the bass is boomy. Moving the third sub around or eliminating it is really the only way to adjust the bass. If you don't engage the 965 subs you will still have mains that are bigger than the system 9000 mains. The biggest advantage to using the subs is that you get more LFE without sacrificing headroom on your amp since most of the power being supplied by the amp can now be dedicated to the mid and upper range speakers. If you have a receiver, it might be able to send a lot more power to the mains without getting close to its limits.