Massive sound at a low price
I bought this sub to extend the low end of my HS7's. I use them purely for entertainment. I was first considering a more expensive KRK sub, but it would've been overkill for my purpose. Here's my experience with the Sub10 after a day of testing:
When I took it out of the box, I was pleasantly surprised by the build quality. It's on par with the Yamaha HS series. This sub will shake your house if you want to, it goes low and LOUD. The sound is punchy and smooth.
However it does have a couple issues. The volume knob is not exactly linear. It gets really loud by turning it up just slightly. And like others have reported, the built in crossover is not that great. Using it makes the sound on my main monitors tinny or 'metallic'. You can bypass it by plugging everything to inputs (XLR & 6,3) as they're soldered together. I think it's better that way.
If you're even considering about adding a sub to your system. This is a great option! You won't be disappointed. Unless of course, you have money to blow on a higher end one.