Subwoofer

Highly recommended as long as you do the management properly.  Look at the MiniDSP 2 x 4

Very timely thread!  I am thinking about adding a sub to my Sonus Faber Guarneri Evolution monitors.  These have a -6db response at around 43hz and starts to roll off at 72hz.  My room is small (10x11, though the back wall is only 3' high and opens up to my family room) and I do not listen at high volumes.  The sub I am looking at has a -6db response of 32hz.  I am hoping that adding a sub has the effects that Stingreen mentioned above.  I have also read that adding a sub will help to "open things up" and "expand the soundstage" though I have no idea how.

 You're not extending the low end, but making mids and treble better....also you change the acoustics of the room by evening out the "bumpy" low end.  

tboooe.....you'll be happy.  Its not necessary to get the most expensive ones you can find.  Even moderately priced subs help and will not destroy your very fine Sonus Fabers.

tboooe,
I swear to you adding subs has "opened things up and expanded the soundstage." I don't know why, but it's easy to hear.
What has been difficult is blending them perfectly!

I do buy into the idea that subs aren't just for improving the bass but the overall experience of the music.  I played around tonight with an AR H6 sub (100W, 8" driver, ported) from our very modest home theatre that was on its way to Goodwill.  Stuck it midway between a pair of Prelude Plus speakers facing right at the listening position (not orthodox sub positioning at all) and approximately on the same "front" plane as the Preludes.  Found some old JPS Labs speaker cables and hooked the sub to mono block amps via speaker input terminals.  Started listening with level and crossover dialed all the way back; gradually increased things.  Found a combination that seemed to blend well...almost couldn't tell the sub was on but for the effect on music I knew well.  Dunno if it was power of suggestion/self-delusion but I'd swear imaging was clearer and sound stage bigger than without the sub.  A fun evening.  Definitely thinking now how nice things will sound with a pair of sealed, better quality subs that are properly positioned.