Alright, got the F12G-SE setup properly with placement and XT32. The sub is behind me. Couple observations:
Integration with the speakers is perfect. No localization whatsoever at the listening position. What subwoofer? LOL
Definitely got a lot less <20hz output than the F15 according to UMIK measurement, as expected. Higher measured distortion down there as a result.
Spikes definitely reduce distortion, especially >40hz. As expected.
Sub can rattle with the spike on tile, especially doing noise test. Sandbag solves everything here.
The enclosure vibration was quite a lot compared to the F15, the enclosure strength doesn't seem to be sufficient when running the AVR's 75db noise...
I kinda wish there's feet base on the amp side, so I can experiment with upward firing placement... Also, Brian, you need better quality control for the feet base/screw-in, they are not leveled enough and they need to be screwed/glued in themselves to the enclosure. I had to "pull" a base out a bit...