We're confusing some terms here. The Denecke AD-20 outputs a S/PDIF (digital) signal through a coax cable (and also optical, but that's not part of this discussion). Line-in, by definition, is an analog signal. To clarify: you should be able to send the AD-20s digital output (S/PDIF over coax cable) to any device that accepts a S/PDIF input via coax - using whatever connector is appropriate for the recorder, i.e. RCA, mini, XLR, proprietary 7-pin, etc. If the recorder does not accept S/PDIF inputs via coax, i.e. if it only supports analog inputs, then no - this is not possible.
The M1 supports this connection with its proprietary 7-pin cable. Is that how you've connected your AD-20 and M1 in the past, with a 7-pin cable on the bottom of the unit? (as opposed to the stereo mini line-in on the top of the unit)