I'm looking to upgrade from the JBLs (which I got for $130 for the pair) because of lack of imaging.
The Axioms look like a move in the right direction.

I'm looking to see if anyone has experience with the JBL S-series loudspeakers that may be able to draw a rough comparison between them (the JBL S310's, specifically) and the M60's and/or the M80's.
Thoughts, comments, etc would be appreciated.

My current listening space is finished basement with full carpeting (including stairs). It's an irregularly-shaped/non-symmetrical room approx 14x18x7ft.

Thoughts/recommendations on which M-series model to try?

I have yet to even hear a pair of M60's or M80's and I'm positive that the JBL speakers come nowhere close! If the M80's sound very close to B&W 703's, (thats what others have said in here) then M80's sound amazing!

I'm guessing here, but I think the M60's would do well with your room dimensions.

I selected my m60's after listening to 703's, the price was one factor, and I think the m60's sound every bit as good or better than the 703's, in my opinion.