James,
I upgraded from nothing. It was a return after a long vinyl hiatus for me. A few months ago, I upgraded to a ZYX Airy3, which was done at the same time I upgraded my table & arm. (see my system page, "Spencer's Funhouse" for details) Therefore, it's tough to draw conclusions about the cart change alone. For more than the Shelter, I think ZYX Airy 2 is worthy of consideration. So are the carts from Jan Allaerts, but you're getting into some higher $.
The 501 was sold to my buddy, slipknot1, who also had a Denon 103, as well as a Dynavector cart. Cheers,
Spencer