I listened to a pair of Q40 about a month ago. I found them to be quite good for the price asked. However, I do prefer the Pro 900 by quite a large margin. To me, it sounded like the highs just weren't "there" for me. The bass was okay, the mids felt a little so-so and the highs were not to my liking. I'm sure though that, just like with every other headphone, this is a matter of getting used to the specific sound signature.
That being said, the Pro 900 has lovely highs and lows - IMO really remarkable - but does not suit every music style out there as it has a rather metallic sound to it. I think it's lovely for jazz, EDM and sometimes even classical and rock-like music like RATM and such. It is the best headphone I've heard to render sounds like this:
I also think the Pro 900's have a slightly overrated bass reputation around these parts. While it is great and emphasized by the headphones, I find that the bass of headphones like HD25-1 II and D2000 are more impressive when EQ is applied. The HD25-1 for example, can take as much as my gear can give - and still asks for more, even after being turned into a leaf blower or a hair dryer.
In short: I wouldn't buy the Pro 900 for bass alone, but for the rather unique sound signature as a whole. If you want bass, I think you'd do better with other headphones and a good EQ.