I've listened to them both. I own the 975's. I think they basically sound the same at lower volumes. It's when you begin to crank it up that the difference shows. The 975 givesyou simply more of everything the 965 gives. Larger sub and the 12" radiator in the 975 provide louder deeper bass. The additional midrange helps those frequencies when driven loud. The tweeters are identical.
I am using only the the ones built into the 975s. In an ideal world I would probably have a stand alone but the amount of bass these babies put out has kept me from feeling like I really need one thus far.