I'm sorry if this is common knowledge, but recently getting into IEM and needing something ultraportable and well isolating, I seem to find entry level IEM starting at $399. If there's anything cheaper I'd be glad to hear, and if not I would still like to hear all of the good options. I would...

Long story short, things I'd like to have: Near-zero output impedance and enough power to run most full-size headphones, ie. compatible Low distortion, flat frequency response, low noise floor, no potentiometer rustling. ie. transparent Separate volume adjustment for headphone output and...