OK, a quick update:
Was able to spend a little time measuring/EQing today, here are the results.
* Denon AVR-X2000 - Audyssey XT, not XT32
* Samson GoMic - no calibration via REW or otherwise
* iNuke nu1000DSP driving the DIY 1260
* Existing BIC Acoustech PL-200 that I'm attempting to integrate with - no DSP
I quickly discovered the myriad problems with integrating such dissimilar subs when I placed the test box. I ended up level matching at 32.5Hz, which was half way down from a large peak the PL-200 gave me at around 37Hz. As I'm new to all this it took a lot
of time, research and trial-and-error. But what I have now sounds very good to my ears.
Black: DIY 1260 alone, no PEQ, no Audyssey, sub trim set to 0 (middle), appears my room is the reason for anomalies at 37Hz...
Green: DIY 1260 alone, after 45 minutes adjusting PEQ, still no Audyssey and sub trim 0
Purple: Both subs, Audyssey engaged, sub trim bumped to +2.5
Still have some tweaking to do such as re-running Audyssey (hopefully doing something about that 37Hz point), but the current setup sounds really good to me and shakes most of the house at our max listening volume (not near reference).
DavidK442 also asked about vibrations. I played many scenes from Godzilla (2014), the Pod Rising scene from War of the Worlds, and the first 30 seconds from Edge of Tomorrow with a nickel balanced on edge in the center of the top of the sub at various volume levels including much higher than our normal. The only time the nickel fell was during 2 of the 6 runs through the EoT lead-in. This sub is in our living room over our basement, it appears that depending on where I stand/sit there are less vibrations in the floor. The sub itself doesn't appear to vibrate enough to topple the nickel.