I'm not a basshead, but it doesn't mean I'm not entitled to my own opinion. I also own a pair of XB500s, and while not as refined as the M100s, I feel like it represents better value. I enjoy the XB500s about as much as I enjoy the M100s with EDM (just about the only genre I like the M100s with). I also owned a pair of D2000s, very bassy and one which which I quite enjoyed. If the D2000's were an 8/10, the M100s would be about a 4/10. They aren't even in the same stratosphere. Obviously not portable, but the price is comparable. I've head the D2000's bass described as boomy and uncontrolled, but if they are considered boomy, the M100s bass would be purely mud.
Also, I never claimed these should sound like an open back headphone, but the vents make them semi-open, and I would have expected them to have a little bit more air in the soundstage. Reviews (like from headfonia) pegged them as soundstage monsters for a closed/semi-open headphone. I found them merely average, even compared to fully closed headphones. They're only slightly wider than the M50, even though imaging is about on par. The similar closed-but-not-really-closed headphones D2000s sound MUCH more open, vibrant, lively.