However, if what you are seeing is at least partially due to the negatives being low in contrast, that may be due to the fact that the film is designed to be developed to a lower contrast.
Double-X
is designed to be developed to lower contrast -- Kodak's recommended developer (for cine negatives) is D-96, which is a low contrast developer. However, like most (all?) cubic grain films, it can be processed to higher or lower contrast by using different developers and developing longer or shorter time. Few of us use D-96 when shooting Double-X in a still camera, and we use times (from the Massive Dev Chart or other sources) arrived at by others developing the film for still negatives at more comfortable (for printing) slope or CI. When processed at box speed in something comparable to D-76, Xtol, etc. you should get negatives that look almost indistinguishable (at least by eye) from "normal" process Tri-X, HP-5+, FP4+, or even Fomapan 100 or 400 negatives.
Muddy negatives can also come from overall fog -- perhaps due to excessive age, poor storage, a translucent storage container, darkroom not fully dark during developing tank loading, and so forth. We'd need to see a photo of a strip of the negatives, showing the rebates, to say more about that.