Here's my 2 cents, after owning a Tomahawk for 2 weeks, and just receiving a Pico, both used:
First, I sold the Tomahawk after receiving the Pico, but only because I really want/need the DAC of the Pico, and I ran into a good price on #220, which looks like the day it was made (thanks Andychen!).
With my shure SE530's, the Pico might just sound as nice as the T-hawk, then again, the T-hawk might have the slightest edge. With my Senn 580's, the Pico is better. That said, the T-hawk holds it's own for a 2xAAA powered amp!
Final words - Tomahawk obsolete? Not a chance! Brilliant, wonderful, juicy, gorgeous with Shures, and likely other IEM's, decent even with my 580's, 300-400 hours of bliss from 2 x AAA batteries? I'll own another one. I'd have kept the one I had, but $ are too tight right now. I don't care how good the P-51 is (I have not had the pleasure), the Tomahawk has a place in the world. It's a fine amp!