While the HD 25-1 II is a good sounding headphone, I would characterize its performance as "very good mid-fi," while my experience with the Amperior reached what I would consider "entry-level audiophile" performance. The Amperiors are the best sounding supra-aural sealed cans I've heard to date, handily besting the DT1350, V-Moda M-80, and, of course, the HD 25-1 II. I love these headphones.
Previously, I've felt the HD 25-1 II had a bit of an edge to them, in the case of the Amperior the highs are simply clear and articulate; there may be the slightest edge to the sound, but it's not bothersome in the least. Sure, they don't manage to pull off the airy, spacious highs of reference quality cans; and neither do they deliver the liquid coherence and transparency that a full-size, open headphone can. But for this type of headphone (small, sealed, portable) they are remarkable.