I've got Yamaha HS-5's in a small room and found I needed a sub. I wasn't hearing ANY of the bass stuff going on and when I'd bring the song out to a car or stereo system, I was shocked every time over how off the mark I was getting. I don't turn it up much, just to the point where I feel it a bit and it's helped quite a bit with mixing. I suppose with bigger monitors I wouldn't need it, but that won't be happening until I move and have a bigger space for a studio.