From what you say I would agree that Digby seems to be the dominant dog.

The major step you can take here is give up taking the dogs for walks for two weeks.

It sounds drastic but to concentrate on the points I gave you at first they will not miss it. They will have enough brain work to tire them out and it will make the rest of their lives sooooo much better for them. It is also a natural behaviour - unless they accept the leader they are not included on trips away from the den, hunting, defence, scavenging or play because they are not trusted in their given role.
Garden only for two weeks and work on the ignore factor and when praise has been earned for a given task, and only then, make sure that Digby gets it first as well as dinner etc.