IME, subs do 3 or 4 things by design (depending on how they're implemented) and a fourth by default:
1) They provide sub bass below 30hz (some say 20hz, as a practical matter I'll say 30hz). To buy a sub that does this well is a very expensive proposition - I'd argue that it's well above your budget. The good news, for a music only system, you don't need this. Even pipe organ recordings have little meaningful info down here (at least the ones I own). By the way, even the subs providing sub bass poor performance (i.e. 30+% distortion which is pretty common at these frequencies) may provide a sense of enhanced sounstage. It's a bit of a mystery to me, but I noted this effect even when I used subs that were "bad" for sub bass.
2) They can provide bass below 100hz. You can actively cross a sub(s) to replace the bottom octave of your monitors. A good one will probably improve this portion of the frequency range. This, of course depends on the monitor in use.
3 - maybe) Depending on the main speaker, even a bad sub will remove the bass load from your main speaker and allow it to perform better, if you actively x-over high enough.
4) A sub allows bass placement near the walls, (and/or digital room correction for some models) which will almost always provide smoother bass performance than even a super expensive speaker placed out in free space.
!) The thing subs do by default is create a real task to achieve smooth integration. There are tools to help you get this result, but they're way over budget.
Bottom line: At $1K (if you're careful with your choices) and (way,way if you like) up, a sub can do great things. At less than that budget, they're much more likely to do harm than good.
IMO and IME.
Marty
