Monitor enthusiasts: musical subwoofers?

What makes a musical sub musical? I am considering the addition of a sub to my monitor-based system, but for the most part I find subs intrusive and difficult to "blend". In selecting a sub for use with a monitor type of speaker (rather than a large floor stander), what do you prefer? Stereo subs? Multiple smaller drivers? Any thoughts/suggestions/philosophies appreciated, Jb3