Here is a simpe one, is the original poster less happy with the sound from the speakers or the LFE? If LFE performance was fine, i'd say sure buy towers. If they are unimpressed with the LFE and the speaker sound is OK, no amount of tower is going to fix that. I went to a 15" sub with Def Tech bookshelfs, and then went towers and large center. I'd take the sub over the towers any day. Id actually take the large center over the towers too, unless we are talking 2 channel music.
For me, having a wimpy sub in a large room with my 7ii's crossed at 60 or 80hz is not the fix. The LFE channel can fix itself with a large sub, and handle deficiency from the towers, the towers cannot fix LFE channel deficiencies.