I've had the 10x5, 20XL, 17Dmk 2 and now the XX-2 mk 2. They are indeed better as you move up the ladder. The 17D is pretty special though, it might beat the treble, it is soooo pure and extended but it may be just a little light sounding because of that. If money was tight and you can live with a little light sound in the bass then the 17D is a winner, but the XX-2 is more well rounded.
Audiomax mentions Shelter carts. I had a 90X before downgrading to the 20XL (bad move) In my opinion the 90X is better than any of the Dynavectors I have tried. It has a real magic that none of the Dynavectors quite have. Though it ought to since it retails for about 1200 more than the XX-2. A more fair comparison might be with the XV-1S.