Okay, as I suspected, even if you don't have the correct adapter, you don't have to go out and drop $40 on the Radio Shack one.
After a little digging I found this thread:
http://www.pcenginefx.com/forums/index.php?topic=2960.msg37827#msg37827You guys are reading WAAAAAYYY too far into it. Everything inside these systems runs at 5v. They use your average run of the mill, dime a dozen LM7805 type linear power regulator. This is connected to a heatsink. You need at least 7.~v input to produce a stable 5v output. Most power adapters are at least 10v to compensate for power fluctuations (your 110v a/c wall socket).
You can feed up to 30v into most 7805 varients. What happens? The extra voltage is turned into heat. If the heatsink isnt big enough, the LM7805 will shut down after a while (they are internally protected). Worst case is you'll fry the 7805 and/or the system fuse. 9v, 10v, 11v, 15v, doesnt matter. The important thing is the adapter provides enough amperage, and polarity is correct.
So essentially, as long as the polarity is right and plug fits, anything in the 9V-12V range that's putting out a minimum of 1000mA (1Amp) or more will power the system no problem. Excess voltage is turned into extra heat, and excess amperage is just not used by the system.
Someone does mention later in that thread that Duo-R/RX may not have internal fuses, but I can't confirm it anywhere else.