I think the D100 ADC sounds pretty darn good, personally. There are a at least a handful of options:
[1] Denecke AD-20
It's a pre and ADC, so I wouldn't even really consider this one. Besides, I think it sounds a little tinny, but don't know if that's due to the pre or ADC in this unit. I personally like the D100 ADC better.
[2] Graham Patten ADC-20
I just don't like this ADC, it sounds too digital to me, kinda artificial sounding. Again, I personally like the D100 ADC better.
[3] Apogee AD1000
Great deals to be had on these, but they're getting old and reliability may be an issue to consider. Like the modSBM-1, this + the MP2 gets you close to a MMe or V3.
[4] Apogee AD500
Same age/reliability issues as the AD1000, although significantly cheaper.
[5] Edirol UA-5 w/ coax mod
Easily run as a standalone ADC. Price is right. Sounds great, I think!
However, I personally would go for a sixth option:
[6] Edirol UA-5 w/ coax and pre (clarity or warmth) mods
I'd drop the MP-2 outright in this scenario and just run SCX-1 > UA-5 > D100. Selling the MP-2 would get you into this scenario with no money gain/loss. From what I've heard second-hand, the UA-5 with the pre mods sounds great. I like the UA-5 pretty well w/o the mods, so I'm sure it sounds really good with Doug's pre mods.
I think there are some UA-5 pre mod samples floating around here and/or at the Oade Bros page.
Let us know what you decide!