Don't mean to nit-pick, but to my knowledge the acronym "PMP" usually stands for "Portable Media Player." This most commonly hints at units with much larger LCD screens, HDD's and function sets when compared to your every day DAP. The acronym "DAP" stands for "Digital Audio Player." This is what you were referring to in your OP.
As for sheer sound quality, it's difficult to compare. Your DAP would not only be amplifying the source signal, but it'd be creating it! So, are we comparing the amplifier sections only? Are we comparing the CDP/computer/tuner/etc. hooked up to your Onkyo with the DAP? The whole package?
That said, portable audio is generally under stress to perform on par with home based gear of similar cost. A general note is you pay for convenience; DAP's are small and convenient. Sadly, you pay more for their small size than their performance. I find even current production PCDP's (portable CD player) out perform most commercial DAP's.