View attachment 40519
Page 73 of the 6700 manual is the configuration I would try. It uses Height 3 for bi-amp. In theory, this should leave Height 1 and 2 available. They don't show the exact configuration you are asking for but if you use the 11.1 "amp assign" perhaps it will offer a bi-amp configuration? The 9.1 configuration above does.
Typically, if you don't stick exactly to the Denon examples for amp assign in the manual there is a good chance it's not supported (but you could try). I found this out with my 4700. Each amp assign feature is designed for a specific feature and if I don't follow the exact connections it won't work. Example: if I use pre-outs instead of speaker connections for Front A-B amp assign feature - it doesn't work. Denon tech support says that's normal.
The assignable channels have strict configurations that don't support pre-outs unless specifically called for in the diagram. Definitely limited.
As far as having three height channels? There are pages of configurations with Heights. They are supported within the assign modes offered.
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