For you Roonies out there right now Roon’s PEQ feature can be used to build an EQ curve based on the Fletcher Munson curve (see Wikipedia) for a specific end point volume level. I used a free app on my phone (Sound Tools) to determine I wanted one for a 60 db volume level. I build an approximation of the curve using Roon’s PEQ features and WOW. It sounded great. I have put in a feature request to the folks at Roon for them to consider offering this in a bit more user friendly fashion where you just enter you desired listening room sound level and it automatically generates the appropriate curve, then you save it as an EQ for use as desired. Hope they do it.
PS- Another solution for anyone with a RME ADI2 dac, there is a built in loudness function.