Hi Wayne, I'm so excited that you found my post! You and Brucek have been the biggest influences in turning me from a REW noob to a (fairly?) knowledgeable user. If you can believe it I actually once ran through both channels of the BFD for a single sub just so that I could access more than 12 filters. I mean heck the line kept getting flatter so it must be better right?
Regarding what you call "global EQ" it's funny you bring that up because I've been wondering those exact same thoughts. On the one hand, what's great about DB's is that adding a second "quieter" sound source will barely affect the existing louder source, so the loudest sub at any frequency dominates. This is very easily seen by looking at the purple combined graph, as it follows the upper envelope of either green or red almost exactly. On the other hand, I have concerns as to whether that means one sub is working much harder than the other over the majority of the spectrum, making the second sub just a "gap filler" for where the first is weak. Something tells me it should be better if they were each flat individually around 69-70dB and then combined to still be flat at 75dB. One of the main reasons I wanted the second sub was to reduce the load on my existing one and I wonder if this global EQ approach has really managed that at all?
My next project will be plotting the corrected response of both subs by themselves with my current filters to see what it looks like, and then with a new bank of filters seeing if I can start over and EQ them separately and still get this good of a response. When I'm done (maybe next week? this is a busy weekend) I'll post more charts and tell the story. Oh yes, and I'll follow the rules for posting charts next time too, my bad on that one.
Thanks for your interest and suggestions. I'm really glad you brought up the global EQ specifically as it just proves there's always more to learn and more to tinker with.