I would love to have the opportunity to compare them, but looking at the specs and components, there is no reason to believe either the Sierra 2 or the New Philharmonitor would have an advantage over the other. They seem pretty equivalent, and I, personally, believe both Dennis Murphy and David Fabricant (Ascend) are fully competent at developing a design that realizes the full ability of those components.
The New Phil has a 5.5" woofer vs the 6" in the Sierra 2, but they spec out at 45Hz and 46Hz (respectively) for the -3dB point. The size gets offset by the quality of the Scan Speak driver Dennis uses. Most likely if you are spending over $1000 for a pair of BS speakers, there would be a sub picking up around 60-80Hz, so its probably more about midrange differences than bottom end.