Alan P and mthomas47, thanks for the input. I appreciate it!
Here are my thoughts on next steps.
1) Do a gain matching sanity check with current 2 subs using my RS SPL meter within 1” of subs. I will try pink noise and a couple specific frequencies, like maybe 50 and 70 Hz, with both sub trims set to the same level and Audyssey off. I’ll do this in the front and back of the subs. If the X-12 is outputting a significantly higher SPL, this might confirm my observation of its working much harder than the LFM-1, suggesting that it is at a higher gain although its gain knob is set very low vs. the LFM-1. On both subs, the driver and ports are downfiring, BTW.
2) If I buy the 2 additional Outlaw X-12 subs (sale ends today, I believe, so I will decide in the next couple hours
), I believe I will need to reduce gain on both of the current subs because they are currently calibrated to -7.5 and -8.0 dB with the Editor app, and adding 2 more subs will probably take me out of the trim range.
3) I will do a gain match check on the 2 new subs to hopefully verify that each tracks SPL with gain setting the same way. I will set the gain to the same setting used on the current X-12.
4) If the Editor app sets the 2 new midwall subs to a lower trim than the front subs, I can just leave it that way (knowing that the midwall subs are being taxed less), or I can adjust the midwall sub trim to match the front sub trim, knowing that I have just boosted the overall sub level some amount. Some iterations could be required to get all 4 sub gains matched to a level that allows adequate trim range for the front and midwall sub groups. BTW, with the current front subs, I typically boost both sub trims 4-7 dB, depending on content.
Seem like a decent path forward?
Hopefully, SubEQ HT will do a decent job of aligning the timing of the front group and midwall group together, since each sub in a group will be within a foot of each other vs. the MLP (the Editor app set the current front subs to 13.3 and 14.1 feet, BTW). I realize that I will have no way of knowing precisely without REW, miniDSP, etc.