at853s > battery box > 1/8'' female/male > ad-20 > digital connectiong cable(the optical out w/ 7pin on the other end) > 7 pin > d8
I think your problem is the 1/8" female/male connector.
the 1/8" inch jack on the AD-20 is an S/PDIF output,
not an input. the only inputs on the AD-20 are the XLR inputs. so, with no audio going into the AD-20, of course you're not seeing any levels on the D8. you need a 1/8" female to two XLR connectors... I know Len at Core Sound sells this cable, and I'm that there are other places to buy it as well. or you could just order the connectors and solder it together yourself.
hope this helps.
edit to say: nevermind, it looks like you got this figured out. you can ignore what I wrote above...