Hi Fred,
The Advents were my first home speaker purchase in the seventies. I still have them, and they serve as my computer speakers today, driven by a an older Yamaha receiver. I'm not sure how many models they had back then, but I've got the two-ways with the ten inch woofers and those unique orange tweeters. As Tom said, I believe they were called the "large Advents". I had to have the foam surrounds redone once... they still sound wonderful!
As you can see by my sig, I own the Epic 80/500. Assuming you're talking about the same model Advents, making the absolute ROUGHEST of comparisons
, I'd say they would equate mostly to the M50's.
That being said, I would still recommend as Mark and Tom have, that you start with M60's, or the M22/sub route... with a slight preference (IMO) to the M60's, and adding a sub later. Sirquack has indeed compared M60's to B&W 701's when he was considering Axiom, and thought they were comparable. In fact I've heard they compare to the whole 700 line. Axioms are also comparable to the Paradigm Studio line.