Okay, I'll explain the basic reasoning and some specific products that meet the requirements.
1st, realize that it is far more expensive/difficult to produce a full size(floorstanding) speaker that is of low resonance(from the standpoint of the cabinet itself). As you increase the size of the cabinet, you have to use proportionately more effort to reduce the cabinet vibration. Also, the LF output of most floorstanders is not very impressive compared to even basic good quality power subwoofer systems.
2nd, realize that most people go around looking for speaker by listening to it - and trying to find their preferred frequency response curve. This is a waste of time and effort because you can, with proper equipment, adjust this curve to your preference, and as such, it will be a better fit for your tastes, since you were able to shape the response to exactly what you prefer. But, very few speakers are suited to being adjusted to your preference, because most speakers have substantial driver related resonances, deviations and cabinet resonances. You need a speaker that is, at least, mostly neutral to begin with, for you to be able to successfully adjust it's response to suite you ideally. This means, also, it does not matter if you like how the speaker sounds when it is not adjusted; because what is important is that it is objectively neutral.
3rd, if you use this speaker mid or far field, the off axis response should follow the on axis as closely as possible; you want the room ambient sound to match the on axis sound, this is critical for sound quality, as is demonstrated by credible perceptual research time and time again.
One of the lowest priced speakers I know of that will fit this bill is the B&W 705. This speaker has lower cabinet resonance than most in it's price bracket(its not a totally neutral cabinet - but it is a vast improvement over most). It has superb drivers with low energy storage, and a very flat frequency response, on and off axis. The response off axis is extremely well behaved, and similar to on axis, even up to about 10kHz, and still decent above 10kHz, compared to most other speakers. Yes, this is a bookshelf sized speaker. But it's small size also offers less cabinet surface area to vibrate and radiate sound energy. But this is also a benefit -- with this speaker you get a superb midbass and treble module. Now, we can pair this with a set of high quality stereo subwoofers. You can also use this speaker nearfield with no problem, as it's midbass and treble drivers are located very closely to each other, and as such, will integrate even at 2' distance from the speaker with no problem.
It is said time and time again that subwoofers never really integrate with speakers for music purposes. Well, this not true as a rule, it's just true in most cases because these people don't have a clue how to properly integrate subwoofers seamlessly. You really need to use stereo subwoofers, and you need to cross the subs low enough compared to the point where the subwoofer itself starts to have resonance issues. The bigger the sub, generally, the lower in frequency it will start to have issue, even though this does not have to be true, but it is based on the typical construction/design of these units.
You can use the subs under the bookshelf sized speakers. You can use short stands or some other spacer between them to get the right height, if space is at a premium. Or just use normal stands and place each sub within 1/2 wavelength of the crossover frequency you use to integrate the subwoofers to the mains. With these B&W speakers, I would cross around 80-90Hz ported, or 90-100Hz sealed(if you plug the ports, which is what I would prefer). For the subs, I recommend a pair with quality along the lines of at least the entry level $400 SVS offering. This is just a suggestion. Ideally, you would have some cabinets built to spec and have a DIY pair of subwoofers. You can get far more performance for the money using this route as compared to any commercial offering.
Now, I mentioned proper integration. A reciever's internal crossover is not likely going to cut it. And you can't just run the mains full range and then blend the sub to them with the sub's crossover(though this is a popular method). You must use a precision device such as a Behringer DCX2496. You can get Behringer, or you can get much more expensive alternative makes, if you are not comfortable with Behringer equipment. I personally have no problem with the DCX2496; I have two of them on two different systems and they perform transparently. This device has very powerful variable crossover abilities. It also has very powerful parametric and shelving equalization abilities, that will let you adjust your speaker's response to your preferences. It can be linked to your PC with a serial cable and you can use it's wonderful GUI software interface to adjust/configure the unit. Or you can use the unit's front panel - but I highly prefer the GUI. The equalization panel lets you literally draw the response you want by dragging your mouse around on a graph. Very nifty.
So, following the recommendation above, you would have a very high quality neutral mid-treble module mated actively to a pair of high quality LF modules. Using the DSP hardware I recommend you can integrate the mid-treble and LF modules seamlessly, and adjust the response to your preference.
Do not neglect acoustical treatments. These are CRITICAL in any configuration or with any speaker system you get/use. My favorite commercially available unit are the Ready Acoustics Chameleons. These are very reasonable cost, and very high grade cosmetically. They have powder coated metal frames and the actual panels are covered with a high grade of suede-like micro fiber; the grade you find on better quality furniture. They are about $130 each last time I checked. If you place your speakers 2' or closer to the wall, behind, you should use 2 of these panels per speaker, to eliminate most of the reflection across a wide bandwidth from behind. This is the absolute minimum treatment. You should use more to optimize the room acoustics, but this is a starting point that you can work from.
For the amplifier, I highly recommend a high powered professional a mplifier. These are very reasonable in cost and very high in performance. The Yamaha P2500S would be perfect to mate with the B&W speakers. It also produces no fan noise, unlike most pro amps, and this is one of my main reasons for recommending this unit.
These are just some basic outlines; please ask specific questions for a better explanation of any issue(s) mentioned above.
-Chris
