Importance of a subwoofer?

[Onset to putrefaction]

I was wondering how important it is to have a subwoofer.... Right now i have 2 different large D condensors (not a pair of the same) 3 sm57's an audix D6 and 4 sennheiser e604's.... Would you guys suggest i get something else before the sub? like a pair of small D condensors?
The fostex monitors i have now have ok bass response, but getting good sounding bass is kinda hard

thanks