Having had my mom's dachshund here quite often I have the feeling that it is alos a territorial thing. The dog may be old but the cats know it's his territory. It will take some time until they have made it clear that it's their territory as well

Here I can see that Amber feels very insecure because of the cats- much more as if she comes in a restaurant or so.

Plus there is these translation problems: a friendly wagging of a dog's tail means something different in cat language

Can't wait for pics