I talked to someone at Sound Devices today. They recommended using the balanced XLR-out's on the Pre's with XLR-F to TRS 1/4" cables into the MixPre-6's combo jacks (assuming I don't need them for other inputs). The TRS connectors are useful in this case, as the MixPre-6's combo jacks are not wired to carry phantom power to TRS jacks (they only give phantom to XLR's, just like some other MFG's), so using them here prevents accidentally adding phantom power to the outputs of the preamps.
I also realized when I was talking to them that the MixPre-D and FP24 have meters in dBu, but MixPre-6 is in dBfs (even though the MixPre-6 manual uses dBu's throughout---confused yet?). So, when I did a practice run the other day, I was running my external preamps way cold. Interesting that the singer preferred them to the MixPre-6's transformerless pre's.
Now, if I could just find some low-profile 12" 90° XLR-F to 90° TRS snakes instead of the extra 50' long snakes I had on hand to connect the units...