Re:Sonar realtek hd audio problem!
If the crossbleed is very subtle, as you say, then it's not the What U hear-, or Stereo Mix-setting that causes it, I guess (??). Those settings give a real double tracked, loud result.
Then it might be genuine crossbleed, and AFAIK that is due to the card itself, it's poor quality (and some question marks again).
SONAR PE 8.5.3, Asus P5B, 2,4 Ghz Dual Core, 4 Gb RAM, GF 7300, EMU 1820, Bluetube Pre - Kontakt4, Ozone, Addictive Drums, PSP Mixpack2, Melda Creative Pack, Melodyne Plugin etc.
The benefit of being a middle aged amateur is the low number of years of frustration ahead of you.