I pulled the trigger on an Amazon warehouse deal on a fiio e11k. Despite the description of possible cosmetic imperfections, the thing is in perfect condition, and more importantly it is absolutely perfect with my PSB M4U1. I wanted to get an amp before a better cable, and I'm so glad I did. The bass boost is absolutely perfect for the PSB, as it just barely adds some more emphasis and presence to the music, but rather than narrowing the soundstage and bleeding, the soundstage is much improved, and if that wasn't enough, the amp eliminated the shrillness I found on some songs that had less dynamic quality whether bass boat was on or not, meaning less fatigue and better sound!