Hi Paul, looks like we were posting at the same time this morning.
My box modeling of this driver indicates it will also make a great mid bass driver with an F3 in the mid 30's in about 12 L ported.
The distortion measurement I did was really loud and the driver had zero reinforcing help from an enclosure! I could see and hear the driver in distress under 50 or so Hz. It was hitting its mechanical excursion limits for sure.
I think Parts Express is going to pot, or they are smokin too much of it. Hope they make things right for you. Last time I had an issue (bent frames on DS90 drivers) their customer service got all pissy and made me take pictures for them before they would issue a RMA.