1. It may be counter intuitive but "center speakers" are a compromise. The best approach is to have three identical speakers across the front soundstage (left, center, right). The "center speaker" concept became popular because many people could not fit a center speaker that was the same as the left and right and they needed something that was horizontal.
Putting a traditional speaker sideways can create issues with the way sound propagates in the room. A well designed "center speaker" has the drivers in a pattern designed to mitigate that issue. But the best solution is to use the same speaker for the center as the left and right, in the same orientation as the left and right speakers.
2. Tower speakers. The advantage of a tower speaker is they can play louder than a bookshelf speaker. In some rooms when the sitting distance is more than 15 feet from the front speakers, that extra output can be important. No matter what size speaker you use, you will set it to "SMALL" in the AVR and direct the bass to the subwoofer. So their ability to play lower is not their advantage.
Your listening distance is not so large that you need the extra output of the tower speakers, but if you prefer them for their look, you can certainly use them.
The downside is that they cost more, and then you are less likely to have a center speaker that is exactly the same as the left and right (though some people use a tower for their center, if space allows). If you use tower speakers for the left and right, you could still use the bookshelf speaker for the center, or use the "matching" center channel, your choice. (If you were using tower speakers because you require their increased output, then using the matching center channel might be better than a bookshelf speaker because in the case of SVS, the official center channel ALSO has a little more output than the bookshelf....but, again, your room isnt likely to need that.)
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If you have extra budget, my first choice would be to dedicate it to a larger subwoofer rather than tower speakers. Unlike the main speakers, where the most of the sound goes direct from the speaker to your ear, the subwoofer sound has to fill the room and most of what you hear is bounced back after filling the room....and you have a large open space, so you almost can't go too big with your subwoofer!
The other place I would spend money before buying towers, assuming you don't have budget to do everything at once, is on acoustic panels on the wall behind the seats. This will greatly improve the sound and clarity of the whole system by greatly reducing reflections back to the listener.
GIK Acoustics is a good source for panels (though DIY is an option). I would put four or six of their 2ft by 4ft by six inch panels on the wall behind the seats.
https://www.gikacoustics.com/product/gik-acoustics-244-bass-trap-flexrange-technology/ But if you don't have space for the ones that are six inches thick, then the ones that are four inches thick are still very useful. And of course they have lots of options with printed patterns on them, etc, if plain panels don't fit with your taste.
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Personally, I would invest in all the above items BEFORE adding in overhead speakers. And, frankly, while the Prime Elevations speakers are very nice for overhead speakers, you don't actually need to spend that much to get good overhead sound, if budget becomes an issue. Something like the NHT Super Zeros are cheaper and would match well with the system, as would some of the Yamaha "outdoor" speakers which can often be found for 99 dollars a pair, and have mounting brackets that let you easily point them toward the listening position.
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Finally, since even if you could listen to speakers at a store safely and responsibly it is still better to hear them in your home, I think you are a GREAT candidate for these internet direct companies that have a generous return policy. SVS is one, but not the only one. And even Crutchfield is pretty good about returns. Last time I checked it was like $10 plus shipping to return a bookshelf speaker. (Verify this before assuming I am right.)
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Totally unsolicited advice: If I was outfitting this space, I would probably buy five Elac Debut Reference bookshelf speakers (
Debut Reference) which are within your budget. Some people don't like these for home theater, saying they are not exciting enough, but they strike me as very neutral and refined and should still have plenty of output. And I would get the largest ported sub I could afford from one of the internet direct brands, while setting aside a few hundred bucks for acoustic panels on that back wall.