Can I use Channel 6+7 to Bi-Amp the front channels, while still using pre-outs for Dolby Atmos height speakers on my Concert AVR or Maestro M9?

Yes.
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Within the setup menu of the AVR 7/9 or M9 – you can select what channel 6 and 7 will be used for.
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Within the speaker type menu, scroll down to “use channel 6 & 7 for”
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You can select “Biamp L+R” and still be able to use the pre-out’s for the Dolby Atmos Height channels to send to a separate amplifier.
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Concert AVR 7/9 product info can be found attached to this article.
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Maestro M9 manual also attached.