Follow up RE: Headstage 4t (not extra-bass version, no DAC version) - the bass boost has good impact and I like the two levels available, but the SZ2Ks aren't rattling like they do with the C5. Much better than using my X3ii's EQ, and MAN! it's so slim. No more struggling to fit my stack in a pants pocket if I want portability and convenience. I imagine I'll forget the visceral feeling of the SZ2K properly amped and go from an EXTREME BASSHEAD to a moderate basshead with wooden cups over my ears. In about an hour I'm going to see how it boosts my Senn IE8s while I listen to Skepta's latest.
Conclusion: Headstage Arrow 4T is not an extreme basshead amp, but does give your already bass-capable headphones an excellent boost and if you don't need that tooth-rattling (this is literal, this tooth thing) bass of the SZ2K + amp that can do it, the Arrow is no slouch.