The Space Box contains four discs of prog rock and art rock, as well as trance-inducing Kraut rock, from the early '70s. Most of this music was inspired by the sonic experimentalism of late-'60s Pink Floyd -- it builds on the long, free-form coda to "Interstellar Overdrive." There are subtle differences between the bands -- for instance, Hawkwind tends to lean toward hard rock more than their contemporaries, who explore psychedelia and classical music flourishes. Even though the set is well compiled and contains some fine songs (Faust and Gong sound particularly good), there's no denying that there is a limited audience for this, even among prog-rock fans. It's experimental music that is oddly limited, working the same vibe, if not the same sound. If you're not a fanatic of space-rock, then the four discs of The Space Box will simply be too much to digest.