If a subwoofer can't get below 20hz (sbwoof, sbwoof), it is not a subwoofer, but a lowly woofer (woof, woof).
No specs posted, no measurements on website...could imply that it has as much extension as a woofer, not a subwoofer (In other words, the specs are so bad that they are embarrassed to post it).
If you want a 10inch sub that hits below 20hz, (qualify as a subwoofer to begin with), the ELAC Varro RS500-SB is 1200 bucks. You won't be getting that for 300 dollars!
Other popular 12inch or above options that "qualify" as subwoofers are Rythmik, HSU, SVS.
Smallsub exceptions: Rythmik also has a dual 8 inch. KEF has a dual 6 inch (that qualifies as subwoofers).