the range of application volume for the best quality is about 50~60%, but wasapi deactivates applicationn volume and gets to 100% automatically.
I've seen variations of this "50 - 60%" reported elsewhere as "optimum for sound quality". With digital audio that does not apply gain, this is simply wrong thinking. Back in the days of analog, when just about every level control had a gain of greater than one, a setting of 75% was a good rule of thumb for unity gain (no adding or subtracting of volume) and therefore best for sound quality. This is true on most professional mixing boards as well.
But with digital audio on computers, the top of the scale on most volume controls is simply 100%. Anything lower and the signal will be digitally *lowered*, when means more calculations and ultimately means less dynamic range being sent to the DAC or sound card. JRiver's volume control definitely operates this way.
So set your volume controls to 100% in the application for sure. Probably in system volume too if it operates the same way. On OSX the system volume control definitely adds no gain, so it should also be set to 100%.
Brian.