Well, this is going to be a bit tough...
especially wanting to get a subwoofer for ~$150 - that's going to severly limit the sort of quality you'll be able to attain.
What I would suggest is that you forego the center speaker altogether and focus on getting a decent pair of bookshelf speakers and a sub for your $500 total budget.
If you purchase speakers that image well and have wide, even dispersion, they can create a very convincing "phantom" center sound. An actual center speaker will, of course, perfectly lock dialogue to the screen, but a center speaker is also the only speaker in a traditional 5.1 setup that is not - strictly speaking - necessary. Get a front pair that images very well and you will not miss that physical center speaker!
Another concern was your plan to put your center speaker inside an entertainment cabinet of some sort. That is a really problematic placement. Don't get me wrong - it is very common and I fully understand the desire to do it from a look perspective. But in terms of the havoc it wreaks on your audio quality, it's something you should try to avoid. Naturally, not having a physical center speaker at all solves that issue!
Now, focusing on your front L/R bookshelf speakers - will they have more of an "open air" setting around them? Or will they be placed within some sort of enclosure as well? And even if they are not within an enclosure, will they be placed quite close to any walls?
I'm sort of guessing that they will.
If that's the case, you'll want to look for speakers that allow for adjustment. Several speakers have "boundary compensation" and "treble adjustment" switches. Failing that, a sealed speaker rather than a ported speaker would be preferable.
Most speakers have a port or ports - that is, a hole of some sort in the cabinet (often in the back). That hole allows the speaker to use the cabinet itself as a resonance chamber (think the body of an acoustic guitar or a violin). That allows a small speaker to have more output (louder) at lower frequencies than a sealed speaker. But it also means that the speaker needs more room to "breathe". Sound actually comes out of that port, plus air moves in and out of that port - which can cause noise if that air movement is impeded. Finally, there are some ported speakers that come with a plug to essentially turn them into a sealed speaker.
Your room size is medium, which means you also need fairly decent output capabilities. You don't want to be struggling to hear your new speakers because they are too quiet or get "screechy" at louder levels.
When it comes to the subwoofer, I'm of two schools of thought for a budget like yours. On the one hand, you can spend almost all of your budget on the sub and get a pretty good sub - although you're still likely to want to upgrade it in the future. On the other hand, you can get a very inexpensive sub that "isn't offensively bad" - which is about all you can ask for in the ~$200-ish or lower price range.
If you'd prefer a "pretty good" sub, I'd point you at this HSU STF-1:
http://hsuresearch.com/products/stf-2.html
It doesn't play super-low (gets down to about 35Hz...don't believe the 25Hz spec on the website, cause it ain't happening in the real world!
), but it plays with good transient response and quite decent output, so in terms of the "quality" of the bass that it produces, it's a pretty good sub. But it's also $350 and there's shipping on top of that, so you've pretty much blown your budget and you'd have to settle from some pretty cheap bookshelf front speakers!
If you'd rather stay on budget, I'd point you to this Onkyo SKW-204:
http://www.amazon.com/Onkyo-SKW204-Reflex-Powered-Subwoofer/dp/B000HMLP5A/ref=sr_1_1?ie=UTF8&qid=1299401182&sr=8-1
It's only $130 shipped, and out of all the really cheap subs out there, it's easily my favorite. "It's not bad!" is my constantly surprised and bemused response whenever I hear this little guy in action. Onkyo includes this sub in all of their HTiB (home-theater-in-a-box) packages, and it's a big part of why Onkyo's HTiB systems are the only ones that do not make me feel sick to my stomach if I recommend them.
This is no "earth-shaker", and it can't possibly compete with a really purpose-built subwoofer. But it's surprisingly ok for its price and actually pretty impressive if you're coming at it with the right sort of expectation level. For ME to say, "it's not bad" means that most people would think it's downright "good", so I can recommend this little Onkyo sub, and honestly, with your budget, I think it would make more sense and be the better way to go overall. Either way, if you keep with this home theater hobby, you're going to eventually want to upgrade your subwoofer! With this Onkyo, you're barely out any cash, you can put the money you save towards some better front L/R bookshelf speakers, and compared to what else you could buy within your budget, it will at least do its job and not totally disappoint you!
So that's the sub out of the way, let's take a look at some speaker options.
I do have to say that, in this low price range, the Behringer B2030P Studio Monitors are about as accurate a speaker as you can find anywhere. But I hesitate to recommend them to you for a couple of reasons. Mainly, they're quite large - way bigger than most people expect them to be. And honestly, they're pretty ugly (IMO anyway
). They're built for the purpose of being used for work in a studio - pretty much zero thought was put into making them fit a home's decor.
I'd say your ears did you proud when you preferred the Energy speakers over the other Best Buy options. While I wouldn't hold up the Energy C-Series as the epitome of great sound or anything, they are at least designed using the scientifically-based research that came from the NRC and gave birth to so many well-known and respected speaker companies such as Paradigm, PSB, JBL, Athena, Energy and Axiom.
I'm going to stick with that "Canadian sound" and recommend Axiom's (axiomaudio.com) excellent M3v3 speakers to you. I've been very, very impressed with Axiom's newest v3 tweeter. It offers wider dispersion and higher output capabilities than the older Ti and v2 tweeters - which were already good. But the newest v3 tweeters offer better imaging, which, as I mentioned, is going to be a key characteristic in choosing speakers for you.
Now, the standard bookshelf M3v3 are ported speakers. As such, they sound best when they have some distance between them and any walls, as I talked about above. They will function just fine so long as they have a minimum of 2 inches of space behind them. But with so little space, their mid-bass is going to sound a little bloated due to their ported design.
If you are going to have them placed very close to a wall, I would recommend that you go for the on-wall version of the M3v3 speakers. As the name suggests, they literally mount on the wall with a unique bracket. You actually mount the bracket on the wall and attach the speaker wire to the bracket. Then the speakers themselves just snap onto the bracket, rather than trying to attach speaker wire to the speaker cabinets - which can be a pain with on-wall speakers.
For the price range, for your needs, and for the sort of sound quality that you already prefer, the Axiom M3v3 speakers are a superb choice, IMO. It helps that they come in a wide variety of mounting options - including the standard ported bookshelf, an in-wall version, an on-wall version, and in-wall/on-wall hybrid version and even and outdoor version!
The wide and even dispersion of the newest v3 tweeter creates a very convincing "phantom" center image and makes a skinny, placement-compromised center speaker unnecessary. And the output capabilities mean no problems in your medium-sized room. They're a good-looking speaker - if you want, they are available in a HUGE selection of optional upgrade finishes for additional cost. Axiom's upgraded vinyl finishes are only $30 extra on a pair of M3 speakers. Real wood veneers costs an extra $160/pair, so that is out of your price range.
The Axiom M3v3 are a big upgrade to what you are considering. Best of all, in the future, if you decide to go for a much larger 5.1 or 7.1 sound system, they can serve very well as your front L/R speakers, or they can easily move to the surround or surround back positions. They also have enough bass output on their own that they can serve as excellent 2nd room speakers.
The little Onkyo SKW-204 subwoofer will fill in more of the bottom end for movies and TV and deliver some nice rumbles and explosions that the M3v3 on their own can't quite reach. For music though, try the M3v3 on their own as well as partnered with the SKW-204. You might find that you like the M3v3 without the aid of the subwoofer for music-only purposes
Hope that helps!
