after extensive use of my petite with my new AKG k271s (55 ohms, but very hard to drive properly due to its sensitivity), as well as my HD 650s,i have come to the conclusion that the petite SHINES with low impedance phones and iems, but is only so-so with high impedance and sensitive phones.
For example, i compared my 271s with MAX volume over my 5.5 ipod, and MAX volume over the Petite (playing Khachaturian Violin Concertos and Lil Wayne's new album), and I dont think I heard a significant difference in volume or SQ. However, using my akg k81 djs (MUCH easier to drive) and doing the same thing, the petite at around 3 o clock showed SIGNIFICANT differences in bass response, soundstage, and overall depth over the ipod headphone out. Same with my sr60s, night and day differences between unamped and amped.
I agree that the Petite is indeed the Tomahawk killer, in that for low impedance phones and iems, it is a great, if not one of the greatest performers, in the portable headamp biz. Just dont expect similar stellar performance up the HP impedance chain, however.
Now i have to find decent portable amplification for my 271s, as they are my current workplace phones (after experiencing its lush midrange, i cant return to my k81djs, my former office weapon of choice)