I have only free aired my x8 but it's not like you need 10000w to get it loud
I also don't have a sa8v3 but the v2 is VERY low end friendly. The v1 was more traditional and I've been told the x8 is more traditional
I prefer the traditional sound more but that doesn't mean they won't get low
I would do the x's just because it gives you the ability to up power if you ever wanted to.
Devils advocate..... if you do sa8's you could go to 6 or 8 if you ever wanted more output for a similar price to the 4 x8's