Well, its really only your opinion, needs, tastes that matter.
Could I build a better one, IMO? Yes, BUT, with that hard limit, I would've built an incomplete system.
Since that Denon is video pass-thru only, you could most likely have saved by getting an Onkyo equivalent for $150, or better yet, find a used SPDIF receiver for say $50.
$550 will get you 3x Ascend 170SE, or various studio monitor monitors, or other value rich speaker choices for the front stage.
You can take the remaining change, +$100-$250, and apply that to a subwoofer such as Dayton, BIC, superceded Velodyne, entry level eD, or DIY, etc.
For the two missing surrounds, if completely unwilling to spend more, Id look for anything cheap on the classifieds, ebay, craigslist to handle them in the meanwhile. $20. Or just as well would I compromise in not having surrounds until I can afford perhaps a bit better.
5x KRK ST6 is $495, OTOH. 6x Behringer 2030p is about $390 I think. Even storing away the extra (which might come in handy one day, anyhow), its still tough to beat.