I've seen quite a few reviews of the BX8s and most have been critical of the unpredictable low end. The SP5s and BX5s, in contrast, have received mostly very positive reviews in terms on their mids and highs (see their SOS or Future Music reviews), but the low end's definitely weak. Adding a sub to one of these smaller models produces a very workable low-cost system, which I've been using for over a year now. With some careful integration, the freq response across the spectrum is pretty flat and predictable. From all I've seen and heard, I'd choose the 5"+sub over the 8" model any day, but then again, I generally prefer the flexibility of a 2.1 setup in my quirky (untreated) little studio.
J.