My assumption is you are tracking at 48k/20bit with the XT20?
If so then I wouldn't change anything..

Just let the Marantz do the 44.1k/16 bit conversion.  They must
be happy with their results. And as long as you don't notice any
artifacting or weird stuff.. your audience won't either.

Although you only get 18 bit resolution if you digitize using the
Korg A/D convertors you can still feel secure that you are
recording at the highest resolution your current setup can
produce.

Rich
gorDesign solutions