2late: I agree that a true sub should be able to hit 25hz with ease but I think you missed the OP’s request of inexpensive (<$300) subwoofer. Try finding a sub for under $300 that can truly reach 20hz let alone 25hz flat, I don’t think they are out there and if it is, I’ll buy two. The Energy is rated -3db @ 27hz which is really good for this price point. Plus the OP is only looking for something to only fill out the bass from the other speakers, not something that will play pipe organ music with clarity. This sub should excel at the OP’s request.
Also, just because a sub does not reach 20hz it does not mean it will be boomy….that would be a function of poor design. There are lots of great subs out there that are only 8 or 10 inches that sound amazing, quick and tight when listening to music, everybody doesn’t need a sub to play big movie explosions at 120db so their windows blow out. Personally, I would take quality over quantity. Of course, 20 years ago, it was a different story.
With respect to floor standing speakers that can truly go down to 40hz, these also cost good money but if they are cheap, you will also get a boomy sound.
But thats just my opinion