I believe they have a slight ported opening, but then again I am not familiar with them. I am also very intrigued by the mad dogs. At $300, they're substantially cheaper than the Sig Pros/T5p, etc.
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They are closed. Stock in vented but I believe that the vents are closed during the mod process. They are incredibly isolating and leak no sound so they are definitely closed.
That said, I know I'm comparing them to open cans at twice their price (or worse), and that's pretty unfair. For what they (Kenwood) are, they're spectacular, and I can readily recommend them as a daily all-rounder. However if your goal is just to sometimes be able to block out external noise (for me it was HVAC), the QC15 are where I'd steer you. More isolation, less money, better long-term wearing. Do they sound bang on perfect? No. But they're good enough when you consider everything they bring to the table.
I like that kind of treble. Although, I find that when I switch to them after listening to a more laid back headphone for a while, like my HD600, the treble does seem a bit over the top on the denons. However, if I just sit down and go straight to listening to my denons, the treble seems fine.