Questions - what is the problem with the current system? Since you are planning on buying another pair of bookshelf speakers (for the rear?) can you just spend another $100 for the tower speakers instead and move the current bookshelf speakers to the rear?
Front tower speakers will give you system more impact and will have higher SPL capabilities than the smaller bookshelf speakers. By the way, that is a great sub and should make a huge difference with most soundtracks.
1. You should move the front bookshelf speakers to the front edge of the stands
2. The center should be just below the TV, can you put the TV on top of the center? The other option would be to get a raised TV mount or a "monitor shelf" to put the TV on and the center speaker underneath
2-Ch (HT L/R): Oppo BDP-105 BD, Adcom GFP-750 pre, Bryston 10B Sub Xover, Bryston 4BSST2 / Paradigm Signature S4 v.2 (L/R), (2) SVS SB12-NSD (Subs)
Home Theater: Bryston 4BSST2 amp / Paradigm CC-590 (C), Outlaw 7700 amp / (4) Def Tech UIW-RSSII (LS/RS/LB/RB), Samsung 46” 3D LCD