I don´t know how high you sit, how tall you are but imagine the ideal scenario would be that all three speakers would be at same height = tweeters at seated ear height. Clearly it´s really tough with tv, but as i said use the height you have available now as you mounted the tv higher. Put the center channel directly under the tv so you can get it higher where it belongs. How you do it is up to you. Either on top of the unit with some DIY stand or the amazon center channel "feets" under the speaker on wall. And yes if you place it on top of the unit, use some pads so it won´t lay directly on it. But for me it would be too low. It´s one of the most active speaker so put some effort!
I have wroted probably 50+ times how to setup it. I think i have to start saving up one of the threads.
- Connect cable to R (Mono) -> Denon subwoofer preout 1
- High level gain to Min (not in use)
- Lo Level gain = volume knob, set this to ~11clock before running Audussey
- Filter Out/LFE
- Frequency Max 120hz
- Phase to 0
Run Audussey but choose 3mic positions only. 1st is the money seat middle of couch usually, the two others are close to it each side. If you have couch against wall make sure the mic is about 0,5meter from rear wall. Use the cardboard stand if you don´t have tripod/boom arm mic stand!
After it has ran, check subwoofer level and report that back. Ideally it´s somewhere around -8db give or take 2-3db`s. We want to have it on the negative trim side so you can easilly tweak the sub from Denon menu and never again touch the knobs. For many the bass level is quite anemic after Audussey so it´s good to have some headroom there to tweak.
Check the 9min video how to optimize things with the Audussey setup cause you can make a mess with it if you don´t care what you do.