I am considering getting O2, but for AH-D600 rather than HE-400. I'm afraid that in the case of HE-400, O2 won't be able to provide enough bass punch and bass impact. C5 has an excellent bass boost so no worries here. Have you tried O2 with AH-D600? It has enough bass quantity on its own, but I...

Given their size, sound leakage, and their non-existence of isolation, I don't foresee myself walking with these outside. So, definitely for moving around the apartment, and primarily for comfortably vegetating on a couch.

Need a portable amp for HiFiMAN HE-400. I narrowed down my possible options to C&C BH, FiiO E12, Little-Bear B-1, Digizoid Zo, JDSLabs C5, and a custom Cmoy (3X Nichicon KW Capacitors, JRC4556 Dual Op Amp, two 9v batteries). Any suggestions which of these bad boys would provide the best synergy...