Several on head-fi.org have done this. You can check over there them and see what they thought of their results.
Can I use Headphones AND a Subwoofer?
Has anyone done this before. I have the Senn HD650s which, I don't think are considered closed. They're not like the AKG K100s either though. I'm missing the sensual bass I used to have with my speaker system and was wondering if this would be a fruitful pursuit. Any thoughts besides, "Your Crazy!"?