Thanks to photos and posts I've gathered that the new GW Agrax Earthshade isn't really a good replacement for Devlan Mud (as you'll see in Sigur's example), but The Armypainter's Strong Tone Ink was said to be. So I bought some in my local shop and tried it out on some spaceships. The two ships to the left have been washed with Devlan Mud, while the three to the right with the Strong Tone ink. I'm happy, and would say that any differences is probably more down to differing amounts of water and such.
There's Strong Tone Dip and Strong Tone Ink - the former dries gloss, the latter is relatively new and I assume it dries matt. Haven't tried it myself. I will have to get some soon though, down to my last couple of pots of Devlan Mud.