Paul,

I have not done the upgrade, but exchanged my 568 for a 568.2 through an arrangement with my dealer.
There is a noticable sound improvement, especially when using Trifield. I would suggest you borrow a 568.2 to 'try this at home'.

Regarding the SmartLink part, I would like to do some speculation:

HDMI is probably going to be the industry standard for digital audio/video connections.
When this happens Meridian will implement it. For the 800/861 this means new cards.
For the 568 it means it will replace the SmartLink connector. Literally, given the layout of the backpanel.
I suppose there will also be a 562v.4 to offer HDMI-switching.

Having an HDMI standard will also mean there are more devices offering this.
So if you are interested in DVD-A, you might get a cheap-ish Brand-X player with HDMI to see if you like the format. If you do, you can go for the 598 which will then probably also offer HDMI-out. Maybe even for video as well...

However: SmartLink is here now, HDMI is just on its way and may take a year to fully break through.

Regards,
Mark