Since you have both rear and side boundary issues, I agree with Gregg (and other others) who have suggested you consider monitors and a separate sub. Since you mention ProAc, you could get a pair of Response 1SC, the matching center, and a musical sub such as Rel or Vandy. It'll bust your budget new, but all are pretty readily available used.
Speakers recommendations when placement is an issue
I could use some help with speaker recommendations-I am restricted to placement with speakers flanking a 6 foot built-in entertainment center and 1-2 feet from the back wall. Currently using Linn LP-12, PSE electronics, CAL Icon MK II. Want to start the upgrading process with floor-standing speakers, however, because of limited placement I don't want to waste money on Avalons or ProAc if I can't make them sing! Any speakers that can provide good sound in this setup? Budget is fairly open. thanks!