Using X-Tol in a replenishment regime would be by far the best approach.
With DDX, the Ilford Technical data sheet says:
AVAILABILITY AND CAPACITY
ILFOTEC DD-X is available in 1 litre bottles world- wide. Used at 1+4 for one shot processing it will develop 16 135/36 films. If reuse techniques are used it will develop up to 50 135/36 or 120 films.
Note the reference to 1 + 4, rather than the more confusing 1:4.
The technical data ("Fact") sheet that covers all the Ilford liquid film developers is accessed from the product page for DDX - here is the link: http://www.ilfordphoto.com/Webfiles/...7124733149.pdf
The description of "reuse" process in the Ilford fact sheet indicates that involves some decrease in quality. It certainly involves a large increase in complexity (developing time is constantly changing).
The replenishment regime for XTol result in a subtle increase in quality, and is incredibly simple.