Though I wouldn't personally give iFi my money, the Zen Air Can will work just fine plugged into the SU-1.
Edit: here are some alternative headphone Amps in the same price range
-JDS Atom Amp+
-Topping L30 II
-Schiit Magni+ / Schiit Heretic
-Sabaj A10h / Sabaj A20h
-SMSL SH-6 / SMSL SH-8s