<font color='#000000'>Pam, this is an issue I've wrestled -- I and my spouse -- so I know where you're at, although for us it was a debate while using Axiom M60s, a different animal than the NHT 2.5i.
My guess is that your room size and movie preferences will have a lot to do with whether or not you want a dedicated sub. The NHTs are supposed to get quite low via their woofers, down to 29 Hz +/- 3dB (vs 37 for the Axioms). While I bought a sub (Hsu STF-2), my Axioms perform fine even without a sub for just about all of our movie-watching needs. Our viewing room is 14'x18'. We mainly watch kids movies, small dramas like "In the Bedroom," comedies, classics foreign and domestic, and a mix of some major Hollywood releases, from Three Kings to whatever. We're not heavy on explosions and action, however. In our case, getting the sub-noises below 37 Hz is great, but frankly, it's a luxury for our room and movie watching. If I were setting up a larger room, or watching lots of action and adventure, I'd view it as a necessity, but it's not.
In your case, depending on your room, I'd think you could easily survive without it -- in fact, unless you really go for the low stuff, you might not be missing a thing. You can always order a Hsu online with the 30-day return guarantee and run some tests. (Try The Lion King enhanced home theater mix and Finding Nemo if you're into that sort of thing.) The thing is, if you find yourself searching out bass-heavy movies to prove to yourself you need a sub, well, maybe then you don't need one.
As far as music is concerned, I would not anticipate that a sub would enhance your music listening. There are musical sounds below 29 Hz, but not that many. And to get these, musically, will a) cost you a ton on the sub front, b) be tough, because it will not be easy to integrate a sub with your NHTs, which were built to handle the sub load. Consider that many subs barely deliver a flat response much below the 29 Hz of your NHTs, and some HT subs that cost $250 - $600 don't get as low as 29 Hz. Your speakers are handling the low stuff in sync with a complete main, and that integration will be hard to beat on music with a sub. IMHO.
Does this mean you should get QS8s for the back? Well, I'm Mr. Renunciation today, because frankly, while I have no direct experience with 7.1, I have not heard ANYONE rave about it yet. Not enough soundtracks have discrete info for it yet on DVD. And what I have heard is that people who upgraded to 7.1 found themselves wondering post-upgrade what the fuss was about. That said, I do covet a pair of QS8s, but as side surrounds, not rears. Were I to create a 7-speaker set-up, my goal would be to have the QS8/quad-poles as the side surrounds and some direct radiating speakers as rears, and then have a (new) receiver that has the option of allowing me to employ the direct radiators (not the quadpoles) for 6-channel or 5.1 audio. Even this is perhaps overkill, as I hear quadpoles can be surprisingly different and good on music (while not designed for it), but were I going nuts, I'd try this out, provided the receiver could handle the switches easily.
My unsolicited advice is to buy nothing, enjoy everything you have, and spend the money on new music or your favorite charity.
Birdman</font>
