Thanks for all the input and opinions gang.
I actually hooked up the old Mark IV rig this evening and played for a few hours with some friends. Gotta tell ya...I wasn't "wow'd". It sounded great, don't get me wrong, but lifting it up to move it where I needed it, hooking it up, and then powering it up and hitting that first chord, even at band volume, was a little anti-climactic...and in a way, I'm thankful.
I was a little fearful that I'd hook everything up and be blown away by the tone so much that I wouldn't want to pull the trigger on selling. And while I still don't really WANT to sell it, I just can't really justify keeping it.
In truth, there were only 2 real differences I noticed in tone, in a positive way, which was;
The Axe-FX is so clear and pristine, that any little tuning nuances that are less than perfect, seem to come through. The Mark IV, not so much...I didn't really notice it.
Also, the Mark IV has a little more "purr" or aggression when hitting big open drop tunes. But again, I haven't upgraded to Quantum, and the settings are not identical.
Never the less, while I love my Mark IV and have great memories with it, I'm afraid it may in fact be time to move on. Especially given all the difficulties everyone seems to be having with unloading their gear.
Thanks again for all the input folks. I'd love to hear more "I have no regrets" success stories