I own a pair of SP-8Bs and am familiar with monitors in the price range of $2500 and lower.
I find that the SP-8B exceeds the quality of
the SP-5B in almost every way, which may alleviate the concerns of some people who regard the latter as a rather two-dimensional, exaggerated monitor. Overall, I find it to be an acceptable monitor for serious semi-professional recording and mixing purposes.
I will begin with the best features. The stereo separation, like in its counterpart, is absolutely superb. The stereo field it creates is quite clear and precise, and thus placing parts of the mix in the field may be done with ease, albeit the "sweet spot" for the field should be much larger. Still, though, this aspect of the monitor is still excellent, comparable to the separation offered by the Event 20/20s. The detail in the upper frequency ranges is almost is good, meaning that few subtle, treble parts will be disappear on these monitors. The actual response of the upper ranges, which is one of the most commended aspects of the SP-5Bs, is also clear and non-fatiguing on these monitors, mostly due to the presence of a soft-dome tweeter. Though slightly more defined, the high frequencies are still not quite as sweet sounding (or inaccurate, for that matter) as on the SP-5B monitors, which will come as a displeasure for those accustomed to the SP-5B.
However, the mid and lower frequencies, both in response and detail, do not fare nearly as well. The middle frequencies are overly prominent, meaning that vocals sometimes sound harsh, though the dominance of the midrange does not create an extremely boxy effect as on most monitors with the same exaggerated response. The detail in this range, furthermore, is only fair, and, thus, it seems to take a great amount of time to tweak sound settings in the midrange effectively. The bass frequencies, however, are portrayed even worse, and, in fact, are the weakest point of this monitor. Though the manual suggests that frequencies as low as 33 Hz are present, I only find the useable, reasonably flat response extending to about 50-60 Hz. Beyond that, the bass degrades to undefined thumps that are quite like those on cheap consumer subwoofers. Yes, the SP-8Bs has a far superior bass end than to SP-5Bs, but it is still lackluster in comparison to most monitors of this size.
Overall, these monitors are worth testing. They certainly have compromises in them in order to trim the price tag, but they perform reasonably well given the price difference between these and most full-sized nearfield monitors (most cost more than $1,000, as compared to about $500 for the SP-8B). If anything, they should serve as a reasonably good set of monitors that can be used to A/B the treble and stereo field details in a mix originally articulated on a better set of monitors, presumably a set to which you will upgrade after you have outgrown these.