Hi Jsman,
Actually, although the 17D3 uses a short cantilever, it is positioned quite far forward, so that the location of the stylus relative to the cartridge mounting holes is very close to the statistical norm of 9 - 9.25mm. From this perspective, mounting shouldn't be an issue. Check over here:
http://www.dynavector.com/products/cart/e_17d3.html.
While some of the Dynavector pages show the engineering drawing along with dimensions, this one does not. You can see from the photo however, that the forward location of the cantilever assembly makes up for how short it is (stylus fairly close to the front of the cartridge body).
Of course, one would expect a manufacturer's more expensive cartridge to be "better", but in this case the 17D3 is far, far better than the price differential from the 20X would predict. I've even commented to the US distributor that I actually prefer the 17D3 to the more expensive XX2, and he couldn't disagree with me.
We both agreed that this preference comes down more down to personal taste than absolute goodness. It's also a function of the components you're matching up with this cartridge, and in this sense, I don't want the above statement to be misconstrued. My 'tables tend to control resonance very well, but a more "excitable" analog rig might not respond as well to a lively, bouncy, cartridge.
I can envision someone with a system that tends toward the bright side as considering the XX2 to be a far superior cartridge. In the context of their system, they'd be right.
In the 17D3 and XX2, it's as if Dynavector took two parallel paths toward the ultimate convergence occurring in the XV-1 series (the "s" and "t" models) - acknowledging that you can't have it all without heroic efforts and a commensurate price. I look at the XX2 as appealing to a fellow who's willing to sacrifice dynamics in favor of refinement. The 17D3 makes the opposite compromise.
While the choice between a 17D3 and the more expensive XX2 seems to be very system dependent, the leap up from a 20X to a 17D3 does not seem to be. You're not sacrificing any refinement in stepping up to the 17D3 from a 20X.
Cheers,
Thom