5.1 is coded with 3 up front and 2 on the side.
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The SB speakers come into play with 7.1 to complete the circle.
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Add to that, ideally all of the tweeters should be at ear level. That's not always doable and most of us have do to compromise a little. For instance in my living room 5.1 setup my surround speakers are to the sides but just a tiny bit behind me. I had to mount them over 6' off the ground too, but I have them on swivel mounts and angled down and forward so they point at the seating position. Not perfectly ideal, but good enough.
You might just have too many constraints to do a proper 5.1 surround setup. I'm not saying give up. Do like I said and contact SVS they'll be happy to sell you some Primes and might even have some suggestions for you. Check some other forums too. Our guys know a lot of stuff here, but most of us are geared more for performance and don't tend to compromise very much when it comes to hi quality sound.