Nice work Steve, though I am more enthusiastic about the Pico, I could not conjure up such a review. It is the excellence of the Pico itself that has made me generally apathetic when it comes to blathering on about mid-level audio gear the way I used to.
My review of the Pico would have been: "Upgraditis Cured."
At one point you say the Pico is "boringly neutral," which I would take as a complement to it. If only all amps could be boringly neutral, then we wouldn't have to worry about the differences of amps, and get along to headphones/speakers and sources. I don't think the Pico is lacking bass, rather, its' neutrality simply shows headphones for what they are. Neither the AD2000 nor W5000 are particularly bassy (something that going balanced helps).
Anyway, the fact that you could even have a reasonable comparison between the Pico and several high-end CDP's speaks volumes to the prowess of the Pico's DAC. It performs at least as well as numerous $1k+ DAC's I've owned. I think your comparison shows that Justin has probably extracted about as much as possible from the parts and form factor chosen.
I don't feel that the low gain setting is an issue for low-impedance cans, even IEM's. Sure, you don't have as much range on the volume knob as you might from an amp with little to no gain (
) but it's not to say that the proper volume cannot be reached. You just need to be more careful in adjusting the volume, particularly with IEM's. The Pico certainly sounds better than amps designed specifically for IEM's, so I think this minor issue shouldn't be a deterrant from obtaining better sound quality.