I also have a Shelter 501 and Denon 103. No denying they are great. That said, both the Denon and the Shelter suffer somewhat on the JMW line of tonearms due to an incompatiblity of their compliance and the unipivot design of the JMW. They can produce very good results, but in the case of the Shelter, 850.00 is a lot to pay for a cart whose excellent performance can't be fully fleshed out on the JMW arm. A case can be made for the 200.00 Denon which is an excellent performer, far beyond its modest price tag.
A compromise, if you wish to persue the Shelter would be to pick up a used one at about 50% of the new retail, which is what I did. I have had good (not great) results with the Denon on my JMW9 arm, but to restate: if you check back through some threads here at AudiogoN, including comments from many who know what they are talking about far better than I do, there is a compliance/unipivot arm compatibility issue at play with this combination.
If I had to describe the 17D in relation to the 20X, I would say the 17D is much more extended in the frequency extremes than the 20. Imaging is rock solid, and in the case of my setup, the side boundaries of the speakers got a whole lot wider, and the music still extended out beyond them. I'm not sure if the deeper soundstage I hear now is solely the result of the 17D or partly because of the new amps I have, but that is much more pronounced than with the 20X (a cart I loved by the way). The short little cantaliever scared the be-jesus out of me at first too, but once you get it mounted and set up the worries subside. I have not had any issues with tracking warped LPs, but then again I haven't played too many badly warped ones.
Perhaps Dougdeacon or TWL will weigh in on the compliance issue with the unipivot arm.