I have heard the RS550's as well as the RS750's (non-Signature) as well as the Axiom M60's (Disclaimer: I now own the M60's).
The best way I have heard the Rockets described is in the context of attending a live music performance...The Rockets sound as if you are sitting somewhere in the 20 - 25th rows whereas the Axioms sound as if you are sitting in the 2nd row. The midrange of the Rockets is very good but they are VERY lacking in upper detail (at least to my ear).
My suggestion would be to try to audition at least the M60's...they would be more of a minimal competitive comparison between either the 550's or 750's.
As to your suggestion for ordering the M80's and holding the others pending a listen, give Axiom a call, I'd lay you odds they could work something out with you...
My .02,
WhatFurrer
