I think it would make more sense to compare the DR-70D preamps against the FP24 as I was saying earlier. Connect the FP24 to the Tascam via the 1/8" input like you would when using your M10. Use the slate tone to calibrate the 1/8" input to -20 and then leave it alone, so you're not needlessly adding any more gain (and noise) on the Tascam. Then of course run one pair of mics through the FP24, and the other pair straight into the DR-70D. Try and set gains as close as you can, but you can always level-match in post later. If you record something soft and intimate needing a good deal of gain, all the better. A loud show could be useful to show what the pres can handle for levels, but it's not going to tell you much about the self noise.
If you don't have any quiet shows to record, just record yourself or a friend playing or singing alone in a quiet room.