The player you use on the PC makes a big difference. iTunes, foobar, Windows Media Player, JRiver, etc.
The sound output mode makes a big difference. Direct Sound, WASAPI, ASIO, MME, Kernel Streaming, etc. Each funnels audio through the computer in a different way, each sounds different.
The amount of CPU activity related to other PC chores makes a difference. The purpose of JPlay is to minimize other activities during audio playback. Basically the computer is asleep while playing music. But this won't affect FR or perception of treble too much.
The DSP applied to the audio in the player or OS makes a big difference. The DSP applied from player software should be high bit depth and good math. If not using an audiophile quality player, avoid using the built in volume control and other DSP effects and volume normalization.
A sense of loss of treble clarity and extension is often felt with poor quality digital upsampling, aka sample rate conversion or SRC. The different types of dithering used to smooth out the upsampling calculations varies in quality and processing power required. Make sure your player is not set for upsampling until you get the redbook files (CDs) sounding good at 40.1k first.
Your USB connection is fast enough for redbook resolution files, which have plenty of resolution to play full treble to 20k, though maybe not as refined as high res. Although the routing of digital signal from USB input to DAC is different from the SPDIF input to DAC. The difference should only be in the amount and type of jitter added to the signal, which should affect refinement much more than FR.
I recommend that you download foobar or JRiver to eliminate the player variable. Both of these sound good. JRiver has 64 bit DSP, including the volume control, plus more DSP options and easier song selection, so it is my preference, but it costs $50 after 45 day free trial while foobar is always free.
Learn about the audio protocols in the PC to see what works best for your OS and your DAC. I think Win7 has WASAPI which is pretty good.
ASIO4ALL is another option. Kernel Streaming is supposedly pretty good too.
https://en.wikipedia.org/wiki/Windows_legacy_audio_componentshttp://thewelltemperedcomputer.com/HW/USB_Audio.htmhttps://msdn.microsoft.com/en-us/library/windows/desktop/dd371455(v=vs.85).aspxhttp://wiki.jriver.com/index.php/Audio_Output_Modes