I think it all depends on whether you use your speakers more for HT or music. My first thought would definitely be get a pair of M60s but if you are a hardcore bass junky then get the sub. Seems like a tough decision since both can provide GREAT benefits but in the end what do you prefer, complimentary floorstanders or a kick@ss sub?
BTW, M60s also provide some nice bass but not sure how they would stack up against your custom speakers. Seeing how none of us have heard your setup, I would still recommend M60s. Have you tried asking Axiom for their advice?
