There's no problem with running a dual opposed sealed set-up. Lots of people swear by them. As you've already noted the Shiva's like a bigger air space. They will have more output at the lowest freq's though if they get the power for it. The Peerless are very nice drivers too, with just not quite as much low end grunt. If you can live with the bigger boxes I think the Shiva X's will be the way to go.