[infinitesymphony]

The sub on the ProMedias is kind of long, but dimensions don't really say anything about performance. Logitech's subwoofers sound pretty boomy, IMO.

Active monitors are just like regular bookshelf speakers in terms of dimensions and purpose, but they each have their own built-in amplifiers instead of having to be amplified by a separate amp (this is what makes them 'active,' rather than 'passive'). If your sound card / audio interface supports it, most active monitors can be run in a balanced configuration, which will improve noise rejection.