The Astell & Kern XB10 is arguably the best mobile Bluetooth-based headphone amp out there. Tiny and extremely light, it boasts an excellent sound and a lot of power. It also has a balanced output and can even play aptX HD. It is the perfect addition to make any in-ear or headphone compatible with Bluetooth. So you have the best of both worlds!
Directly from Astell & Kern authorized distributor, the Headphone Company!