Hey guys,
Thats right, you hit the sweet spot in Hugh's problem. Ethernet crossover cables cannot be used to link a device to a router. Just to add some logic into it, as i know Hugh apreciates the analog part of it :
Crossover cables are just what they are named after. In a normal cable you have 4 wires responsible for data transmission + 4 wires to control and signal. Thats 8 wires total, or 4 pairs as it is seen actually.
From those 4 wires, 2 are for Receiving (+,-) and 2 for Transmitting (+,-). So in a normal configuration SB/Duet to Router, the source receiver wire connects to end transmitter wire and the source transmitter wire to the end receiver wire!

If you put 2 devices that are alike : Computer, Duet/SB or other source, and need them to comunicate directly, you will have to "CROSS OVER" the cable ! So you will switch the 2 wires from one end of the cable to the other 2. Receiving becomes Transmitting and vice versa. In practice, Wires 1,2 become 3,6 and vice versa.
Routers are designed to do that (and a few more things) ! Just re-wire in order to establish a comunication between devices.
Too much geek for ya?

hope i helped more than i scramble...
Now...plug in that Duet and start listening to some Music !!
Felipe