Chris Roemer cleared this up for me so I will quote his explanation below...
"If you click on an FRD file (in Windows) it opens in WordPad, and you can see the 3 columns of data: Freq. (in Hz), SPL (in dB), and Phase (angle) - instead of showing phase running from 0* to 360*, it's shown (by convention - or .frd file specification) as -180* to +180*.
The ES180 (and the DC28) start off in negative territory (ES180):
19.86599 75.37 -11.1
20.19895 75.62 -12.9
20.53749 75.87 -14.7
20.8817 76.11 -16.5
21.23169 76.34 -18.3
21.58753 76.57 -20.1
21.94935 76.79 -21.8
22.31722 77.02 -23.4
The DC200 woofer starts out positive. That's the cause for all your concern.
I've designed hundreds of XOs w/out even looking at step response. I HAVE observed the "impulse response" when measuring FR in ARTA (you actually HAVE to look at the impulse to set your "gating" time). From time to time a batch of drivers will come out wired "backward" (at their terminals). The .frd file is still OK.
To make use of F/Z data (designing XOs) you have to either take mfr. F/Z files and "process" them, meaning creating "minimum phase" files AND using X, Y, and Z driver "offsets", OR ...
you can take your own FR measurements and use YOUR phase data (which includes "time-of-flight"), BUT - you can't change the position of your mic between driver runs or you lose the (proper) relative offset in your phase measurements.
You can NOT just use phase data out of mfr. data files (randomly). You have no idea the conditions (or distances) the data was collected at."
