The Pro model has a redesigned second stage poppet. The original had a common problem with annoying free flows that typically came about after 20 dives or so. The vast majority of original Axis regs have now been refitted with the new style connector. You can check if one has been converted by looking at the seat connector after removing the second stage hose. If the connector is green in color, then it is the new style. The originals were white.
I have one of the very early production originals that still has the original connector. Mine has never had a problem after many dives. It is now used as a deco reg, and offers great performance. I've used it as deep as 115', and actually did a test where I tried to overbreathe it. After more than two minutes all I succeded in doing was getting light headed from the hyperventilation. Several dive buddies also use the Axis for deco purposes. They have proven to be very reliable after the connectors are replaced. BTW, Mares has a standard policy of not charging customers for this refit.
The Axis offers surprising performance for such a simple reg. The VAD really does a nice job of reducing overall work-of-breathing effort, and allows the reg to remain very stable. The purge button is superior to that of the Proton, and the larger casing is hardly noticeable in the water.
Greg