I'm in the middle of looking at new mics, right now all I have are at853s, but I don't think those will stick around more than a few months.
Well, you didn't ask but I'm gonna say it anyway.
If it was me and my budget was tight, I'd sell the MP-2, sell the AD-20 (if you already have it), sell the AT853s, sell the battery box, and:
[1] upgrade mics
[2] buy a used PS-2 (several available right now in the Yard Sale)
[3] buy some line transformers for gain
and run Mics > PS-2 > line transformers > D100. This will improve your sound much more dramatically than adding an outboard ADC and far better than the MP-2 improves the AT853s.
If my budget was not tight, I'd still do the following but my focus would be first on mics, second on outboard preamp (instead of line transformers), and third on outboard ADC.