Thank you for the clarifications. With 8-foot high ceilings your cubic footage of your HT room is 4,320 (considered a large room). With such a wide opening you are looking at over 8,000 cu. ft. (anything over 5,000 cu. ft. is considered an "extreme" room and effects the subwoofer choices). This means, forget completely about the Boston Acoustics sub. It simply cannot fill your space, especially if it is for movie LFE.
As far as speakers, I understand your concern. However, tower speakers often come with outriggers to stabilize the cabinet and you'll have a far greater selection with superior sound qualities.
It seems like you are in the middle of getting your HT room completed. I don't know if you will be having a projector and screen, or are looking for LCD-type video. Nor do I know if this is a separate purchase or is part of your $5,000 budget. If it is for projection, one can get an acoustical transparent screen and have your 3 front speakers located behind the screen (safe for children) and opens up a whole new area for speaker possibilities. If it's a regular flat-panel LCD or Plasma or 4k, I'd still look for towers, but obviously that is your call and I won't go into what to look for.
Subwoofers, I can help you with. If you are at all handy, I highly recommend DIY (do it yourself). One can get amazing subwoofer performance for half the cost of some high-end subs. However, if you don't want to go through that, then look at subwoofers from internet-direct (I.D.) companies. You will need two, and possibly more. The following is your homework assignment

. Take a look at each sub, read the specs, check out the warranties as well as the size and weight. Any of the following subs are considered excellent for their price, while others are outstanding:
Power Sound Audio (PSA):
XS15se (15-inch sealed design) @ $799 (includes shipping both ways):
http://www.powersoundaudio.com/collections/subwoofers/products/xs15se
XV15se (15-inch ported down-firing design) @ $899 (includes shipping both ways):
http://www.powersoundaudio.com/collections/subwoofers/products/xv15se
S1500 (15-inch sealed design just released) @ $949 (includes shipping both ways):
http://www.powersoundaudio.com/collections/subwoofers/products/s1500
V1500 (15-inch ported design just released) @ $1,049 (includes shipping both ways):
http://www.powersoundaudio.com/collections/subwoofers/products/v1500
XS30se (dual opposed 15-inch drivers, sealed design) @ $1249 (includes shipping both ways):
http://www.powersoundaudio.com/collections/subwoofers/products/xs30se
S3000 (new, dual opposed 15-inch drivers, sealed design) @ $1399 (includes shipping both ways):
http://www.powersoundaudio.com/collections/subwoofers/products/s3000
S3000i (new, dual opposed 15-inch drivers, sealed design) @ $1599 (includes shipping both ways):
http://www.powersoundaudio.com/collections/subwoofers/products/s3000i
Talk to Tom.
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Reaction Audio (these are all new designs!):
Gamma 15 (15-inch sealed design) @ $850 (includes shipping):
http://reaction-audio.myshopify.com/collections/subwoofers/products/gamma-15
Echo 15 (15-inch ported design) @ $950 (includes shipping):
http://reaction-audio.myshopify.com/collections/subwoofers/products/alpha-16
Gamma 18 (18-inch sealed design) @ $1200 (includes shipping):
http://reaction-audio.myshopify.com/collections/subwoofers/products/gamma-18
Echo 18 (18-inch ported design) @ $1400 (includes shipping):
http://reaction-audio.myshopify.com/collections/subwoofers/products/echo-18
Gamma 218 (dual opposed 18-inch sealed design) @ $1,700 (includes shipping):
http://reaction-audio.myshopify.com/collections/subwoofers/products/echo-18
Talk to Jeremy.
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Hsu Research:
VTF-3 Mk5 HP (15-inch ported design) @ $799 ($89 shipping):
http://www.hsuresearch.com/products/vtf-3mk5HP.html
VTF-15H Mk2 (15-inch ported design) @ $899 ($109 shipping):
http://www.hsuresearch.com/products/vtf-15hmk2.html
Talk to Dr. Hsu
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Rythmik Audio:
E15HP (15-inch sealed design) @ $1024 or $1174 or $1349 ($1024 is sale price, check on shipping):
http://www.rythmikaudio.com/E15.html
F15HP (15-inch sealed design) @ $1224 to $1399 (check on shipping):
http://www.rythmikaudio.com/F15HP.html
FV15HP (15-inch ported design) @ $1274 (on sale! check shipping charges … other finishes $1339 and $1559):
http://www.rythmikaudio.com/FV15HP.html
Talk to Brian.