MOTM

I use Genelecs in the stuido for monitoring.  However, flat as they 
are, they are missing the entire bottom octave to octave and a 
half.  To fill in the missing bottom I use a Sunfire subwoofer.  
This is the best sounding sub I have ever heard.  It even sounds 
better than my Velodyne in my home theater.  Usually I mix with the 
sub dialed in , but I have calibrated my system so that the sub with 
the Genelecs is flat clear down to subaudio.  Many of my clients 
argue that they want the sub turned up, but this is not a good idea 
for mixing.  I always listen to the mix without the sub as well.  
For playing around with the MOTM I have a pair of Mackie 824s which 
are great sounding as well, but not quite as good as the Genelecs. 
I have been considering adding a sub for the MOTM as well.
You won't realize how much you've missed no matter how good your 
speakers until you add a sub. 
I would heartily endorse the Sunfire, but the Mackie subs are quite 
good as well and certainly more suited towards playing live if that 
is what you are after.