as per the backvent issue,
I too ended up taking them off. there was an increase in bass, a bit on the bassy side for me.
Not to mention, i think it made the clampening effect of the headphones feel even worse. (I'm sure this can be explained)
it does increase soundstage, but I'd recommend the grilling mod too for that. I did the grilling mod and ended up taking the backvent mod off.