That sounds to me like poor mastering, not a specific flaw in either PMP or headphone. Certainly there is not a lot of wiggle room with planars and an unamped portable device. But I do not know many people who want to use planars for portable use - they are large and do not isolate well. So there is something to be said for selecting the right tool for the job at hand.
This is different than trying to assess the relative performance impact of any given part of the chain. It's a matter of appropriate equipment selection for the task.