NHT is my preference also, particularly for music... I've recently auditioned their latest Evolution line and am impressed... of course, that is strictly opinion... Polk's are certainly decent, and may be a little easier for your Onkyo to drive... if at all possible, try for an audition, even if it means a little traveling outside of, uhhh, Dogpatch.
| must I get identical front and surrounds to properly listen to new recording formats ie, SACD and DVD-audio. I suspect 80% music listening |
If you're very serious about SACD/DVDA, you should try to match fronts, center and surround as best you can, within budget... hi-res multichannel is designed to go to 5 matching full-range speakers, but this is impractical for most people... while a dead match all around is not absolutely necessary, I'd strongly advise it at this level.
To get the most out of hi-res with reasonably-priced speakers (ones that won't necessarily handle the lowest freq's), you'll certainly want a sub sooner rather than later... and possibly an outboard analog bass management system, if you want to maintain the most accurate signals (otherwise, you can rely on whatever BM the Onkyo does in the digital realm, with some signal degradation).
Happy hunting.