I think he did test eco. If not, you can do it yourself. The way it works may not be exactly the same for all models but I suspect it would be at least very similar in principle. So in general, eco in auto would kick in when volume reach around -30, +/- a few dB (I guess..), depending on the model number/year. There is likely a delay (at least that's the case for my X4400H) between it changes from on to off and vice versa.
So if you listen with the volume setting at around -30, in Eco auto, the amp may clip more often if you are listening to contents that have high dynamic range. For example, at say -32 to -35, with ECO off, there could be relatively rare moments when the input level would drive the 150 W rated internal amps to clipping. But when ECO is in auto, at the same -32 to -35 range, the amps could have been clipping much more often because ECO On or Off when in Auto is dependent on the volume setting only, that is, during those short duration peaks when the power demand would be much higher, the amp's clipping point could be reached at output level as low as 30 to 50% (or even lower) of the rated output. Again, different models such as your X8500H may operate differently, but I know that's how my X4400H works, and that's why if I listen to low quality material such as most of Youtube's I wouldn't mind setting ECO to auto, but for high quality lossless contents I would always set ECO to off, even though I use the AVR as prepro only.
Actually I just remember Audioholic.com did test ECO as well and it was on a much newer model.. If my explanation is too clumsy to read or unclear to you, try reading the Audioholics review on the SR8015, surely Gene is a much better writer.
The Marantz SR8015 11.2CH Atmos/DTS:X Pro HDMI 2.1 AV receiver is built on a very solid platform with benchmark measured performance. Is this the best AVR for 2020? Read our review to find out.
www.audioholics.com
He did suggest using ECO auto is fine, but in my opinion, it may be fine for contents that have low dynamics but not for action movies or classical music. For contents with high dynamics, if you use ECO auto, you are likely going to experience clipping more often; and if you are sensitive to the resulting distortions, it might explain why you noticed listening fatigue after extended periods at spirited but not overly loud levels.