I know you just picked up the MX-100 and this doesn't directly address your question, but...have you considered a digi-modded UA-5?
You'd have:
[1] a pretty good pre
[2] a nice ADC
[3] the option for future pre upgrades if you buy from the Oade bros
[4] the option to run 3-mic mixes
[5] the ability to go 24-bit when you're ready
[6] capability to output optical to a JB3 if and when your DAT croaks
Blah blah blah. Just a thought.
All that said, I personally like the sound of the AD-20 better than the ADC-20. I, too, find the ADC-20 bright and harsh. But, as already mentioned, the AD20 is a problem because you'd effectively be running two pres: MX-100 and AD20.
If you already own the AD-20, I'd pick up a pair of attenuators and give it a go. If you don't like it, the attenuators are cheap and yo ucan always sell 'em with the AD20.
But if it was me, I'd sell the MX-100 and AD-20 and pick up a digi-modded UA5 from the Oades.
Then again, I'm a big UA5 fluffer around here, so...FWIW.