I've got my 15" Avalanche in a low-tuned ported box right now, and the lows are just insane, even though I really need a sturdier box. Mine flexed so bad it is leaking and basically coming apart inside.
I'm dying to try two of these sealed though, and at least doubling the power. I've seen several people say that the Avas are "supposed" to be insane sealed, but there's only one way to find out. They have an Fs of around 16 Hz.
If I had the room for 2 15's ported, I would do that, but that's not gonna happen in the trunk of my civic. I'm betting that with the right amount of power, in the right airspace, 2 sealed will give me at least as much output as the single 15 ported with even better sq. That would be ideal.
I think, from my very limited experience mind you, that for a single-sub setup, especially 12" and smaller, that low-tuned ported is the way to go. But what does going sealed, while doubling the cone area and the power, give you versus that? That's what I'm trying to find out too. I'll know the answer in the next few months. (at least in respect to the specific woofer I'm using)