I can't make a specific recommendation. Really, you probably should go to a retail store and listen! (It's hard to judge speakers without listening.) If you want to "rattle the walls" and "feel the bass", you're probably going to need a 12" or 15" sub... But, it depends on the size of your room, the design of the particular speaker, and how "loud" you really want to go.
You can get "powered" subwoofers (with a built-in amplifier), and un-powered subs that require a separate amplifer. Since you're not using a "home theater" receiver with a "subwoofer" output, you'll also need a
crossover. Most powered subs will also include an active crossover.
Parts Express sells finished subs and kits (as well as drivers) for reasonable prices.
For your application, you might want to consider a "pro" subwoofer. A professional subwoofer (designed for live music or clubs) will be much more efficient (louder for given power). Typically, they will have a bigger cabinet and larger driver (15-18"), don't go quite as "deep" as home subwoofers, and they aren't as "pretty". You can find pro subs in your price range, but I'm not sure if you'll find a
powered professional sub in your price range.
Musicians FriendOf course, if you buy online you won't be able to listen first.
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A good sub and receiver, mostly, that would be able to handle these 4 speakers and a sub?
You might have trouble driving 4 speakers with a stereo receiver. You'll need to check the receiver specs and the speaker impedance. ("Ohms" might be marked on the back of the receiver.)
When you put speakers in parallel, you lower the impedance... Most stereo recievers can handle two pair of 8 ohm speakers (a 4 ohm load on each channel), but two pair of 4 ohm speakers (2 ohms) will burn-out some receivers/amplifiers. (Lower impedance means you are "pulling" more current, more power, out of the amp/receiver.)
With mismatched speakers, one pair of speakers is going to be louder than the other. But, this shouldn't be a problem for a "party" application.
Another consideration - I assume you want to use the two extra speakers as rear speakers? Depending on the size of your room, sometimes it's better
not to surround
everybody in loud sound... You can end-up with everyone standing outside, or in another room where they can converse!
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It would also be nice to use this setup with an HDTV, but this isn't necessary (I'm aware HD capabilities in receivers would up the price significantly.)
A home theater receiver is going to "expect" a front-center speaker also. But, with Dolby Pro Logic II, you can get some nice "soundfield" surround-effects from stereo music. (And, 5.1 channel digital audio from a musical or concert DVD can be amazing!)