Well try it, it won't hurt and it might work. Another case is you used to use the Speedtouch in bridge mode with your only computer on the network, then your ISP will get the MAC of your computer, might try your computer MAC, too.
BTW I have a Speedtouch 530v6 over here, it works well to connect to my MAC aware ISP with the DIR-655. There's one caveat though. As my ISP recognized my Speedtouch MAC, I can only use this MAC to connect to the service or call my ISP to change my MAC to the DIR which I would not like to do as I still want to use my Speedtouch as a router in special cases. The problem is the Speedtouch has the same MAC on both its WAN (ADSL line) and LAN interface, so if you set your DIR to the same MAC, they can't communicate. This is how to change your Speedtouch LAN MAC address:
- Connect your computer directly to the Speedtouch. As you run your Speedtouch in bridge mode DHCP should be off and you might need to set your computer to static IP, sth like 192.168.1.1 for the default subnet of a Speedtouch, to able to connect to it in the next step.
- A telnet client to connect to the Speedtouch. On Vista or 7 you need to install it from Turn Windows features on or off or use 3rd party client like Putty. Press Win+r and type in cmd then run telnet 192.168.1.254 (the typical IP of a Speedtouch).
- Log in then run ip iflist, the result will show your local interface and a loopback interface and their respective MAC addresses. Copy the Speedtouch local interface MAC address, then change it slightly by adding one to the last byte, something like 00:14:7f:0f:27:74 to 00:14:7f:0f:27:75 (it can be anything in MAC address form but I prefer this).
- Type this next ip ifconfig int=<your local interface> hwaddr=00:14:7f:0f:27:75 . Wait for a while for the arp cache on your computer to be updated and you'll get a responsive telnet session back, type in saveall and you're done.
Now you can connect the DIR with the Speedtouch MAC as its WAN MAC address.
The Speedtouch cmd I typed out was from the top of my head (as I'm on the DIR now) that contains a lot of other router command so they might be wrong but also should be close. You can use tab to autocomplete these commands to be sure.
Hope it helps.