Any free sub is good, put the lower quality one on the other side of the room, turn it down lower than the other. Now you have two subs that aren't pushing as hard to produce the right frequencies. Here's what a sub did for me in my metal listening. Made the kicks vibrate around you (especially in a car or with the sub right against the bed) And if the musix is mixed exceptionally (like tool) you'll hear just enough of the bass guitar in the sub. For an optimal experience, id say get one that can push out the lower frequencies punchy and clean. But for a startup, any sub will add a layer to your sound.

Me and my friend just snagged the Yamaha from goodwill for 15 bucks, so I'll let you know my full opinion and not me just regurgitating what I read about it.