Those are really, really good dude. Perfect dirtiness for Orks, IMO. I also don't think my bases are any better than yours, but if you want to add a little depth to your "standard" (i.e. not the one with the rock, which looks awesome btw), my method is:
1. Take medium gravel that you can get at pretty much any hobby shop and white glue a layer of it to the base, leaving obvious holes and edges without gravel. Once that dries, take the model that you want use and figure out where you want his feet. Then, build up any areas to make it fit right with his soles. Once you are done, prime with chaos black and then paint it with the brown color you use above. That is the first "layer."
2. Next, get some Stirland Mud texture paint from Games Workshop (
LINK) with a bigger brush and load it on the base haphazardly to cover any areas that you left exposed and to build up any areas that you want. It is a slightly darker brown than what you have in the picture, from what I can tell, but the contrast between the two browns will add some depth. That will be the second layer.
3. To even out the two different shades of brown, get some drybrush paint from Games Workshop, like Tyrant Skull would probably work for yours (
LINK), and gently drybrush the tips of the rocks on the base. This is the third layer, and it should turn out pretty nice.
4. Finally, to complete the effect, take the same drybrush paint that you used, and drybrush the boots and up to the knees, getting lighter as you get higher. That will tie the model into the base and make the black pants look more dirty/dusty depending on the color.
5. Add the same brush/flock that you have there and it should look awesome.
The above all being said, your bases look more than fine to me, so I wouldn't touch a thing!