For more information than you can consume regarding headphone gear I'd suggest headfi.org. While researching your question I did discover this post from a headfi member.
"The B&W P7 is designed to work with portable devices, and as such, does not need an amp to sound its best. Unless you really want one, consider not splurging on mid-tier gear for now and focus on something cheaper, such as the Schiit Magni 2 (amp) / Modi 2 (DAC) stack. That will more than drive the P7 to their full potential and still be future-proof for most of your future headphone purchases."
"The B&W P7 is designed to work with portable devices, and as such, does not need an amp to sound its best. Unless you really want one, consider not splurging on mid-tier gear for now and focus on something cheaper, such as the Schiit Magni 2 (amp) / Modi 2 (DAC) stack. That will more than drive the P7 to their full potential and still be future-proof for most of your future headphone purchases."