Before going to a replenishment regime I'd recommend first reading about it as discussed in detail by Kodak in this excellent publication (including how exactly to do it for a variety of developers including XTOL should you decide to go that way). Pros and cons are discussed. See pages 15-17 (this entire publication is an excellent reference though):
Gerald's HC-110 suggestion is worthwhile considering.
DDX is an excellent developer. It works fine at 1+9, but make sure you still have an adequate amount of concentrate in the working solution to develop the amount of film. Refer to Ilford's instructions - although they do not explicitly indicate the amount of concentrate required per area of film.
In general, more developer is better, and skimping is not worth the potential price.