I think you answered all my concerns. It's a difficult decision for me. The difference in price is only $100, but it seems I have more options with the 3600. Now that Denon has introduced a new 2020 model the 3600 may come down even more in the next few months.
I am assuming using as a pre-out only, the heat wouldn't be an issue. I am not looking to any of the Denon's internal amps at the moment, but there is always that option.