Conclusion:
So, switching back and forth between the SP-8Bs and the BX8s, the versatility of the BX8 won me over. Still I think that the SP-8Bs are a little bit better focussed on the vocal range (which probably has to do with a differently chosen cross-over point), so in the end, I kept the BX8s and returned the SP-8Bs.
As I didn't have to pay anything extra, I basically have the BX8s for $650, which is quite a good deal in The Netherlands. For any US inhabitans it's an even better deal, as most stores sell them for $479, which is just an outright steal!
Highly recommended, when buying your 1st or 2nd set monitors, unless you have more than $800-900 to spend. Then you'd be wise to save up a little for some more high-end stuff, like Mackie, Blue Sky or A.D.A.M.