I found something interesting!
I used to connect my DAC through my back-lit mechanical keyboard (simply because there is a USB-A port). Last night I turned off the flashing keyboard light so I can enjoy the music more. Then I suddenly found the noise floor of the amp dropped to almost unnoticeable!
Holy cow! I actually don't understand why, but I felt I missed almost 2 months great music time .....
hp4funyou don't want to plug your DAC through a USB passthrough on your keyboard. That introduces a lot of electrical noise that you don't want coming into your DAC, and probably explains why the sound improved when you turned off the lighting. Try to plug your DAC straight into your PC, preferably directly into the motherboard rather than a USB port on the PC case. The name of the game is to reduce interference from other electronics' signals into your DAC as much as possible.