I wasn't questioning the usefulness of subwoofers, just the "not cheap" admonition.
I'd like to add some subwoofers to my system, but I don't really want to spend a grand on them and was considering the "cheesewoofers" recommended elsewhere on ASR.
Well, you don't have to. Broadly speaking there's two ways to do it in my experience. That is of course assuming you are not buying used or building DIY kits. Both of those are 100% recommended. I'm just leaving them out of the equation because if you're going one of those routes you're probably advanced enough not to need my advice.
There's a cheap (but decent) way and a way that runs you at least a grand.
I mentioned the expensive way for OP because OP seemed to be committed to spending a grand or two on speakers and it would be sort of a hilarious mismatch to combine them with cheap subs. Though it might be fine. Who knows.
Way number one, the cheap way. Get two cheap but decent subwoofers. Good examples would be
those recommended by Wirecutter, like the Dayton SUB-1200. I have personal experience using two of those in a couple of 2.2 setups and totally recommend them as long as you know that you're getting into. They are $150ea and you can usually find some coupons or free shipping deals from PartsExpress on them. I got mine for $120ea w/ free shipping about three years ago but I haven't seen them that cheap in a while.
What will this get you? These will "politely" extend your existing bookshelf speakers down to 40hz or a bit below. Frankly, it is not too difficult nor expensive to design a subwoofer that covers a single octave or so, and most reputable budget subs manage this perfectly fine. They don't deliver a ton of definition, nor "slam", but used properly they don't really do anything wrong. They just sort of make your bookshelf speakers sound like they play deeper than they do, and that's okay! That's actually a neat trick for $300 total! Bonus points if your bookshelves play deep enough to be crossed over at 60hz instead of 80hz.
People crap on these affordable subwoofers, but in my opinion that's because they crank the volume up too high attempting to get some bone rattling bass at which point they (like any overextended subwoofer) will not sound great. Used properly they
extend, not
offend. I really do recommend them.
Way number two, the pricy way. Get two decent subwoofers. $400-$500 seems to be the price range where subs start getting really good. Lots of power, definition, etc. These are generally internet direct retailers like Hsu, Rhythmik, SVS, etc. Now you're cooking! You'll hear those callouses on the performers' fingers during those super-funky slap bass solos.
I lied, there are actually a billion ways. There are many ways to skin this cat. You can find a zillion internet testimonials from folks who are happy with carefully tweaked and positioned single-sub setups. That's just a bit harder and less convenient. If you're building a home theater system, a single monster sub might even be preferable to two modest subs. For music I'd always rather have the duallies though.
Opinions vary widely here! Ask ten audio nerds and you'll probably get ten answers!