Do you have UPnP enabled on the DIR601?
That is where I would look if any incoming router port were to become enabled without my having explicitly enabling it.
UPnP is a security hole, and if you don't have an explicit need for it, I would suggest that you disable it in your router even if that is not currently the cause of your port 80 leak.
Here is a screen shot showing where to find the UPnP control:
If your Actiontek modem is not in bridge mode, you should also check to make sure that UPnP is not causing the problem in that device (I don't have a screenshot available for that device)--
We can never have enough of nature.
We need to witness our own limits transgressed, and some life pasturing freely where we never wander.