After using the SubPac S2 all day yesterday, here are some more thoughts:
Overall, I'm VERY impressed.
Listening to music you're already familiar with is a wonderful adventure. You'll hear things you've probably missed for years, even in you own music.
Using headphones with the SubPac is a fantastic monitoring setup, but I also loved using the Subpac while listening to my monitors. The Subpac functioned exactly like a subwoofer, the beats were pounding but the house wasn't shaking and the neighbors never heard anything!
This device will be an incredibly useful tool for me.