The best I can say about most cheap subwoofers is they make bass. The bass won't be the deepest, most powerful, or the clearest, or blend all that well with most speakers, but all subs make bass. Better subs, like the $449 Hsu Research VTF-1 MK2, generate deeper, less distorted sound, so you can actually hear distinct bass notes, and can play louder and fill larger rooms better than most cheap subs.
So my expectations for Dayton Audio's $79 Sub-800 weren't high. Still, I can't say enough nice things about Dayton's B652 bookshelf speakers. There's no better speaker you can buy for anywhere near its $34.80-per-pair price, so I thought maybe the Sub-800 would set a new standard for under $100 subs. I didn't have a set of B652s handy, so I started out listening to the Sub-800 with a pair of $70 Sony SS-B1000 speakers, a Denon AVR-1912 receiver, and our Oppo BDP-93 Blu-ray player.
Starting out with CDs, the sub's clarity and punch were well beyond what I've heard from other budget subs; the Sub-800's 8-inch woofer and 80-watt internal amp were doing a hell of a job. The Sonys are just OK for cheap speakers, so I switched over to a pair of Pioneer SP-BS22 bookshelf speakers ($130). They're a lot better, and supported by the Sub-800 the Pioneers sounded like much bigger speakers. Pressed into home theater duty with the "Hunger Games" Blu-ray, the speakers and sub, which together retail for $210, sound considerably better than most under-$500 sound bars or Bluetooth or AirPlay speakers, including the $600 Bowers & Wilkins Zeppelin Air. The Pioneer/Dayton combination sounded bigger, clearer, and more dynamically alive. As for the bass, well, the Sub-800 makes real subwoofer bass; the Zeppelin Air or a Zvox pedestal sound bar aren't in the same league. The Sub-800's bass has a remarkably solid kick, and goes nice and deep.
It's not the fairest comparison -- I was using a $500 Denon receiver for these listening tests, and that has to be factored in. Alternatively, you can use a cheaper receiver, like the $170 Yamaha RX-V371BL or a $100 Sherwood RX-4109 stereo receiver. Since the Sub-800 is self-powered, the receiver's quality won't make a difference in the bass you'll hear. In any case, the Pioneer/Dayton advantage over more expensive sound bars and iPod speakers will still loom large.