I took my HD500 in after upgrading it to 2.62 and measured the frequency responses of the Global EQ. I used an Audio Precision to generate a 100mV RMS signal into the Guitar input (set to Normal) and listen to the SPDIF output. I used a blank patch and panned Paths A/B to full L/R, respectively.
Things aren't nearly as interesting as the FX EQs (plotted here for reference) because there are no % labels. Line 6 properly labeled the actual filter parameters and, based on my measurements, they did a pretty good job of getting them right. L/R balance is perfect. No surprises, although my measured Q was x2 what their label says. Not a big deal in my book. Still, attached here are half the graphs. Due to upload filesize limits I had to spread the plots out over two messages. The rest of the plots follow below.