Router Port Forwarding not working since Mavericks

I have an iMac sat behind a BT Home Hub 3 (an ADSL router, if you're not aware), with several ports (POP3, SSH, HTTP, etc) from the outside world forwarded via the hub to my Mac. Since installing Mavericks (from Snow Leopard), NONE of these ports are now open.

They're open on the Mac, and another machine on my home network can use them locally, but from outside the network, they're hidden. Shields Up shows them as Stealth.

Things I've tried:

• Rebooting everything.
• Setting the Home Hub up again, forwarding all the ports again.
• Turning all the sharing off in Mavericks and then back on.
• Trying the same services on different ports.
• Trying different services.
• Turning off the Mac firewall.
• Turning off the router firewall.
• Trying the Mac on another IP address and reforwarding everything to the new one.
• Putting the Mac in the DMZ.

I can forward ports to other devices on my network (e.g. my NAS) and they work fine, so port forwarding isn't completely broken on the router

Here's the thing though...

On a completely separate network, on a separate Mac (a 2012 MacBook Pro) that went from Mountain Lion to Mavericks, also behind a BT Home Hub 3, EXACTLY the same thing is happening. No ports can be forwarded to the MacBook Pro, but all the ports are open to the local network.

Any ideas? I'm pretty sure this must be a bug in Mavericks as I can't see anything else it can be.