Done properly a subwoofer will increase dynamics, soundstage and detail. You should not know the sub is even turned on until you turn it off.Concur 100% with this. When set up properly it is wonderful. Just last night I moved my system out of my dedicated room (daughter returning from school for summer) and was listening to Beck's "Sea Change." The bass was overwhelming so I turned the sub off. Things were better but definitely lacking so I went to work adjusting the sub for its new placement and before Beck was finished the sub was dialed in. Now if I turn the sub off it is only noticed in the very lowest register, more of an ambiance thing.
This is with a pair of Proac 1.5s which aren't exactly bass shy speakers. I don't think I'll ever be without a sub in my system again.