I have used the Koss PortaPro, and it's variants, for more than 30 years. But, as small as it is, it doesn't fold flat for shirt-pocket carrying. And it's open-back, so there's no sound isolation.
I'm looking for a closed-back alternative, with bass response at least as good as the PortaPro, with smooth mids and highs, that will be even more compact in my pocket than the PortaPro. I am not a huge IEM fan, even though I own several of them. And I want better bass extension than the PX200.
My Samsung Galaxy Exhilarate Android smartphone actually has more "oomph" at the headphone jack than my original Nexus 7 tablet. It can drive, for instance, the JVC HA-S500 to much higher levels than can the Nexus 7. And the Galaxy Exhilarate is what I'll mostly use the headphones with. I have several portable headphone amps, but mostly don't carry one with me.