Double amping is when you amp something...twice. It's more likely to introduce distortion into the signal chain.
PLYR2 only has USB and 3.5mm input--and doesn't come with 3.5mm cable btw--so the only way to connect it to the Xonar U3 is via 3.5mm cable. This will result in double amping.
Again, I would question the idea of buying a PLYR2 if you're going to be sitting right next to your computer anyway. There's many wired headsets you could buy instead, since wireless is unnecessary for such a setup, and the U3 is going to handle Dolby Headphone already.
PLYR2 only has USB and 3.5mm input--and doesn't come with 3.5mm cable btw--so the only way to connect it to the Xonar U3 is via 3.5mm cable. This will result in double amping.
Again, I would question the idea of buying a PLYR2 if you're going to be sitting right next to your computer anyway. There's many wired headsets you could buy instead, since wireless is unnecessary for such a setup, and the U3 is going to handle Dolby Headphone already.