Can you tell what's different (sounding) between the sealed sub and the OB subs?
Not really. It would take a lot of swapping in and out and re-setup to get to that answer and it really needs to be measured for that. At the moment I'm crossing the sealed one over at 50hz but at a somewhat reduced level (kind of filling in only), and the OB's are at about 80hz acting as the main subs L and R.
The best way to describe it is the OB's are darn near invisible, sounding as quick and direct as the mid/woofer. I know in a technical sense it's not really about speed, but it just sounds very continuous, like it keeps going deeper without any obvious transition. The sealed sub adds just a smidge of that visceral physical impact you can feel that the OB's don't.