The sub is always good to have, if it is a good one and you are able to set it up properly. Besides, it may serve well your upgrades in the future, such as going for a Home Theater. I had my M60s for around a year, before I purchased my REL sub for a 2 channel system. I still have a lot to do to set it up the right way. However, I am not sorry I purchased it, I can feel its presence in several of my CDs, and it is a nice one.

Axiom M60s, QS4s, VP100 Onkyo TX-SR804 Oppo 970HD Rotel RB-1080/RCD-1072 REL Q150E sub, PS 3