If I recall correctly, the AD-20's lowest gain setting is +17 dB. Meaning, you have the gain dial turned down all the way and it's still boosting the signal by +17 dB.
I know that Sound Professionals offers an
AD-20 with a mini-plug input that provides "plug-in power" (I'm guessing around 9V). So obviously, a "mod" for the unit is possible. Also, FWIW, I've read in some thread here at TS that someone did their own mod on the AD-20.
So, cheapest route might be getting the AD-20 used and modifying it yourself, or getting someone else to modify it.
It seems like you could eliminate the MMA6000 from your gear chain since the modd'ed AD-20 would provide plug-in power for your DPAs.
Lastly, poorlyconditioned wasn't impressed at all with the sound quality of the AD-20. So you
might get more pleasing results running DPA4061 > MMA6000 > MD, that using the AD-20. But, that might depend on which MD deck you're using as well...