It sounds like you've done quite a bit of troubleshooting! Not much else I can offer... Sorry.
I wonder why the Denon requires UPnP to be disabled? That's weird.
If you were to buy a new router, you could probably keep using the ZWire as just a DSL modem. There ought to be some way to just use it as a modem, and let the new router act as, well, a router. That might solve the problem. That's what I'd probably try.
I believe that there are two types of DSL networks; Frame Relay and ATM. The modems are different. Unless you're on a very old DSL network, you should be on an ATM. I really can't say for sure. I know that a lot of electronics stores (Best Buy, Staples) sell DSL modems. You could always try a new one, returning it if it didn't work.
Something else you could try - do you have access to another network? A friends house, perhaps? A bit of work to haul the 3808 somewhere else, but testing it on another network would give you more info. If it worked on another network with a different internet connection, then you'd know it was something with your Zwire. If it didn't work at the friend's house, then perhaps your 3808 itself has a problem.
