Ok, I'll bite, who can point point a section (say, a bass hit or something) where they were like "ah HA! that's the difference"
First another note of explanation to set the stage for how I do listening comps...whenever I do comps like this, I load the samples into Audition and then lay the tracks side-by-side so that I can listen immediately without any time delay whatsoever between the two listening samples. I go back and forth from one to the other and back again listening to the very same passage over and over. Then I'll mentally isolate all different types of sounds...I concentrate separately on lows, then mids, highs, vocals, guitars, drums, etc.
For me, these samples weren't necessarily an 'AHA there's the difference'. However, I did notice a clear winner when I selected Source Y in my blind listening test.
Through my phones, Source Y has an overall better shimmery sound that is pleasing and sounds more realistic to me, like it sounds when you're listening live. That was my 'aha' difference that made Source Y for me the winner.
If I'd have to choose a specific 'aha', the biggest technical difference I heard was on the drum kit. I'm not a drummer, but I think it's called the snare drum that I focused on...it's the drum that's the most treble-y in the kit. The sound of the drum hit happens and then there's a momentary decaying of the sound of the drum hit. On source Y, that decay sustains clearer and with better resonance. In comparison, on Source X the sound is more thud-like with less pronounced decay.
Obviously, I'm not saying that Source X's drums sound like a 'thud' to me, but this is a comp and I'm simply trying to provide a word description of the differences I hear. While I did hear clear differences that lead me to choose Source Y fairly quickly, I'd of course never say that Source X sounds bad. The differences I hear are definitely subtle, but enough to kinda make me alot more excited about hearing the forthcoming promised preamp comps on the USBPre2.