My personal philosphy with single speaker design is to minimize electronics for the majority of the spectrum. While one can get drivers (like Lowthers or Fostexes) that are offer good frequency extension, ther eare other factors like tone. I am partial to the "tone" of older alnico drivers (from the 1960s & 1970-s). This means that in the higher ranges and the really low ranges, i have to play games like supertweeters, and (so far not tried but will soon) : subwoofers.

Also, I use only tube amps (cheap tube amps with less than 10 watts per channel power).

However, different people have different approaches: I know MArtin has done great work in actually building lots of enclosures for single drivers.. and trying different methods to compensate for their weaknesses. I thought about building a horn or long pipe, but decided I wanted coherence, and to me, as a layperson, if the bass is coming from behind the cabinet, it lacks that "single driver magic".
Anyway, this is a deeply subjective & personal issue, and each of us has to find our own place!
-akhilesh