Since the Ultra's sub-outs are not full range, using the AVR's multichannel inputs could cause problems; the Ultra's sub-outs are low-passed at the crossover frequency, and I believe are further low-passed at the AVR. Likewise, the main outputs on Ultra would be highpassed. Also, even if the AVR's multichannel inputs can bypass the DSP, I don't know if they can bypass the DAC. (Probably not.)
If you are using the AVR as a surround amp (or pre-main amp), I would just connect the Ultra's audio out to the AVR's audio in. Leave the subwoofer connected to the AVR as is. In this case, however, there is little advantage to using Ultra. If you do not need a display, you can substitute Pro+, Pro, or Mini.
If you want to use the AVR as a power amplifier, connect all devices to the Ultra and connect only the Ultra's audio output to the AVR. This allows you to use the WiiM PEQ for all sources. At the same time, you can use the AVR's sound field correction.
In this case, you may need to adjust the audio delay on one of the devices.
Please figure out what works best for you.