Iirc,
@PENG explained it very nicely. I use eco mode on my Marantz, but I also use a 250wpc amp for my mains. The main reason I do use eco, is just so it runs cooler. I think sometimes too much is made about temps. I have an older pioneer 1019 that I think would blister you if you touched it and it’s always been fine.
I also think OP should try 60-80hz set to small. I think the AVR is struggling.