nad and mistral

im currently biamping nad c370 and 270. im seriously considering replacing the 370 with an lfd mistral. any reason why i couldn't biamp with the 270?

They might have very different family sounds, so the overlap in bandwith between drivers being driven by different amps might lose clarity and coherence. Maybe. I used to own the LFD Mistral SE and it's an unusually sweet sounding solid state integrated. I like it, I recommend it. NAD's I've heard were not so sweet, although I've not owned one.
I would second the LFD Mistral recommendation. I had one for a over a year, and it was my last stop before tubes. The nice thing was that it generated NO heat AT ALL and it drew so little power, I could plug it into a PS Audio P300!

If you are a set-it-and-forget type of guy, the Mistral is a cost-effective option.

However, I have since gotten into tubes, and I haven't looked back!