How hard are these to power? I'm thinking of upgrading my Phonak PFE 112s (another fairly inefficient IEM). From what I understand, the SM64s are pretty difficult to drive, and from searching this thread, I can't find a thorough answer to whether or not they're acceptable unamped. I would be hoping to use these primarily to commute and at work without an amp, or, at best, paired with an e7. Would I be short-changing myself if I dropped the dough on a pair of SM64s?