I wouldn't wander into the idea that one DAC chip had softer bass, that's just too big if audible not to come from something else in the implementation. else it would appear clearly in the chipset's measurements. and there are just too many variables for me to even dare put my finger on only one possible reason.
but about usb I'm 100% with you guys. from the get go it's a bad choice for audio. obviously USB is a convenient universal choice because it's on almost any device nowadays ... but it comes to no surprise that the most convenient, most universal, not at all done for music in the first place, might not be the highest audio quality option ^_^.