C.A., welcome. As Alan points out, forget about the speaker "break-in". Also, your 504 is a fine receiver and a great bargain; it can easily drive the M60s.
As to what could possibly be a problem(beyond becoming more accustomed to the sound over time), Alan's suggestion about checking to be sure that all the drivers are actually operating should be followed. One further thought on that area is to make sure that the connecting strips on the M60 terminals are tight so that both the high and low frequency sections are in fact connected.
Connecting Strips? You mean the 2 sets of wire terminals?
