Subwoofer

I have learned that without proper phase integration with the mains, the sound will suffer big time. When the sub and speakers are in time,the whole system improves across the board... When out of phase,bass becomes slow and lacking in dynamics and musicality.

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!