I personally think that sealed boxes have better sound quality. I really don't think losing 200 watts and half of your cone surface area is going to compare to what you have now. If I was to use just one sub, I would get a high excursion sub like an eclipse aluminum or the like that has a 2 ohm load. I used to run one of those at 1.5 ohm with about 400 watts and it was pretty loud, but I can't imagine it being as loud as what you have now.
