I've read through the thread, and my question, perhaps a dumb one, is basic. Is the DR-70 a viable option for us as tapers?
Most definitely yes! The thing works flawlessly when I run directly into it using all channels.
My issue that I am trying to correct is that when I run my pre in front of it specifically.
I'm confident I'll figure it out one way or another!
Sorry if I missed it, but have you tried running the FP24's XLR outs AND the 3.5mm out at the same time into the 70D?
Seems like you could do that (especially until you figure this out, so you'll have backup tracks). That may help narrow down the problem. Can you do some tests in a controlled environment (i.e. living room or studio)? Play some music with lots of low frequency energy at high volume, and place your mics very close to the source.
Also - do you have another recorder that can take the XLR inputs? You could see if that recorder shows the same odd spikes. If so, then you'll know it's coming from the FP24.