As far as I understand, there isn't really any research showing that phase linearity is particularly audible, let alone important. see:
various statements.
There are no phase measurements of the 8351B available that I know of, and the 8351A is a different speaker.
That said, here are phase measurements of the
8351A,
8361A(same tweeter/mid/amplifier as the 8351B, only the woofer is changed), and
KH420. It does not seem to me that the 8361A/8351B do poorly on this measurement, regardless of its importance.
E: Additionally,
IMD of the 8350 (vs
8361A) seems far worse in the midrange, as expected from a 2-way, and certainly I would be worried about that long before I look at phase.