An observation
I have a Onkyo AV amp, of similar size to that Denon, sitting on an open (on all sides) glass shelf. There is approx 5 cm of free space above the amp to the next self and the TV sits on a further shelf up (top of unit).
I have the amp wired for 7.1 but operate it normally at fairly low volume. This evening I put a temperature probe on the top of the amp and towards the back where the temperature is the maximum. After approx 5 hours of continuous operation the top of the amp was at 50C. For me this isn't a problem because I have no equipment near enough to the amp to be affected by elevated temperatures.
I would however be a bit wary if I had a LCD TV in close proximity. LCD panels are typically rated at 0 to 50C for operation. If the air at the bottom of the TV is already at 50C from the amp the temperature at the bottom of the panel is likely to be somewhat higher. I would recommend checking the temperature at the physical TV/amp interface after a few hours of operation.
With regard to fans - they can be quite noisy and in a quiet environment spoil your listening enjoyment - try just tuning them on and see how noisy they are. Possibly better to find fans that are specified to run at a low speed and add a series resistor in the power lines so that they run even slower. You don't need that much of a air flow to remove the heat if the fans are placed on top of the AV amp - probably at the back of the amp and behind the TV screen.
In the (distant) past I used a 80mm fan spec'd at 1000rpm and then speed reduced with a series resistor to reduce the temperature of a STB by around 15C - the fan was attached to the top of the case with cable ties and using only the existing ventilation slots in the STBs case.
I would also recommend having some space under the amp for the free flow of air. You may have removed some large feet but consider fitting feet that are smaller. In the past I have used something like
Code:
https://www.amazon.co.uk/Rolson-61310-Felt-Pads-25/dp/B00DQ7B7PQ/ref=sr_1_8?_encoding=UTF8&c=ts&keywords=Furniture+Pads&qid=1643160106&s=diy&sr=1-8&ts_id=1938455031
or
https://tinyurl.com/2p9ef8pc
which are self adhesive and can be stacked for various heights. This type of pad is readily available in many "pound" type shops.