Comparison is all we can do when you've not heard something, and I have a lot of things to compare them to. So I won't go into painful detail on it, but I will basically give a summary of similarities or differences based on other well known bassy headphones. All comments are based on the Q40 compared to the headphone listed.
Ultrasone PRO900 - Q40 is less bright, better mids, just as bassy. This was my first thought, was that the Q40 sounded like a PRO900 with the treble equalized down heavily. So if you're looking to save money and get what sounds very similar, this would be the way to go. PRO900 had a slight edge on sound stage.
Beyer DT770 PRO - Q40 is a touch less bright, but the DT770 isn't overly bright either to me. Otherwise, the Q40 had more prominent bass and mids were similar. So the Q40 is a bassier DT770, with a touch less treble, for a bit less cash.
Beyer DT990 600ohm - Q40 has much less treble, similar mids, and more prominent bass, but less sound stage and less comfort, but naturally much more isolation.
Fischer Audio FA-011 - Q40 has less treble brightness, similar mids, and more prominent bass with a similar sound stage and to me better comfort (with pads upgrade) and of course more isolation.
Fischer Audio FA-003 (or Brainwavz HM5) - Q40 has less isolation, similar sound stage, a touch less mids perhaps, smoother treble, and much more prominent bass; similar comfort with similar clamp.
Denon D2000 / D5000 - Q40 has less bright treble, less sparkle, but otherwise quite similar. The Q40 has more prominent mid-bass, but quite similar in sub-bass. Sound stage is better on the Denons, comfort is definitely better on the Denons.
AudioTechnica M50 - Q40 has less harsh treble, similar mids, if not a bit better, more fun bass, more prominent mid-bass, much improved sound stage, and more comfortable to me (subjective though); virtually better in all ways to me for the same cost (including the upgraded pads).
AudioTechnica A900X - The Q40 is less bright, less forward mids, but way more bass in all areas, with a similar sound stage and to me better comfort (with pads upgrade).
Panasonic HTF600 - Q40 has similar treble, both are smooth, similar mids, both have good mids, but the Q40 may have better mids of the two. The bass is similar, but I'll give an edge to the Q40 simply because felt a little better controlled. Isolation is definitely better on the Q40. Soundstage is about the same (again, with the upgraded pads). Overall detail and refinement and build quality just feel/sound better on the Q40.
Sennheiser HD650 - Q40 has more sub-bass and mid-bass, a bit more treble, a bit less mids, less sound stage, less comfort. Only compared due to bass & smooth treble similarity.
Sony XB500 - Q40 has more treble and more mids right out of the box, in a much better way, better sound stage, and very similar bass capability. Obvious upgrade path for a basshead to me.