Well after a few days of A/Bing I've decided to keep the Fischers.
Compared to the SM64s, they're a little brighter and lighter. More treble energy, less low end, mids are similar given the fact that I think the Earsonics mid bass bleeds a bit. While the SM64s are exceptional in their musicality, their dark signature takes a bit of clarity away compared to the FA4. The FA4s bass is certainly lighter but still enough quantity and great texture all the way to the bottom.
Soundstages are interesting. To me the SM64 sounds rounder - like the instruments are surrounding me, while the Fischers sound like the instruments are more in front. Even though the SM64 is a jazz king, the Fischer beats it handily when it comes to classical and still holds its own with my jazz albums.
Tough call overall, but I'm keeping the FA4s and selling the SM64s.