I think I'd try to stay with the BP2X surrounds, both for a better timbre match to the BP8Bs, as well as for some better bass extension that will be closer to the bass extension of your front and center speakers. That way you can use a fairly low cross-over to the sub (80 Hz. or maybe even a bit lower). (Since most receivers don't let you separately set the cross-over frequency for each channel.)
(As an aside, I have the older BP10s for fronts, the old C1 for a center, and a pair of BP1s for surrounds. The BP1's are more or less similar to today's BP1.2s, I think.) When cycling pink noise around all of my speakers, I hear a noticeable timbre difference between the surrounds and the fronts/center. My belief is that this timbre difference would be reduced quite a bit if I had BP2X's.)
I know you wanted to avoid an on-line sub-woofer purchase, but there are an awful lot of people with SVS and Hsu subs on this forum who are very happy campers. They are both very good values, and might allow some money saving. Worth thinking about, anyway.