Hi Music Enthusiast,
Yes I agree these power output measurements can be confusing when some manufacturers state a Vrms output and others give a mW output at a specific resistance.
I'm using Bowers and Wilkins P7s and Meze Audio 99 classics.
The B&W P7s have 22 ohm resistance, and 111bB/V sensitivity, so are quite easy to drive. I'm using these at about 67-80 volume with the Plenue V (out of 140), depending on the track and situation - I tend to listen louder out on the go to drown out other noise.
The Meze 99 classics are 32 ohms and have a sensitivity of 103dB/1mW, and use them at more or less the volume as the P7s - probably even 3-4 steps lower.
So overall there's plenty of volume left for these easier to drive headphones. The thing about output power is to get a good sound it's not just how loud you can get the headphones but whether the quality of the sound is right too. I previously had some Sennheiser HD6XX/650 headphones, which have 300 ohm resistance, and experienced that problem - with some sources I could get them to go loud enough but they would lack a full sound, and come across as thin and missing a bit of life. I sold them before I got the Plenue V though, so can't comment on whether it would be able to drive headphones like those properly.
The B&W P7s and Meze 99 classics are the only headphones / IEMs I own at the moment, so can't comment on other headphone pairings with the Plenue V. But twister6 has written a few extensive reviews of the Plenue R, which has 1.4 Vrms output, and Plenue 2 which has a 2 Vrms output, where he compares how they sound with a range of different headphones and IEMs. For the Plenue R he notes that higher resistance headphones and IEMs don't quite sound right:
https://twister6.com/2017/11/10/cowon-plenue-r/3/
For the Plenue 2 he doesn't mention any specific problems with higher resistance pairings:
https://twister6.com/2017/06/06/cowon-plenue-2/3/
https://twister6.com/2018/03/24/cowon-plenue-2-mark-ii/
If you look across those reviews I would imagine the Plenue V at 1.7 Vrms would sit somewhere between.