Weird subwoofer volume issue on KEF LSX II LT for stereo/mono bass signals. What's happening?

[vroad]

As you can see from the attached graphs, switching to L+R from L only changes main speakers' measured SPL but not subwoofer's on REW measurements.
The subwoofer is SVS 3000 Micro, connected to my KEF LSX II LT via its subtout.

I don't think this behavior is normal.
Denon AVR-X3800H, which is paired with ELAC DBR62 front speakers and SVS 3000 Micro (same sub as the one connected to KEF speakers), increases the sub volume when I switch to L+R as I expected. My AVR-X3800H is callibrated with Dirac,

Also, I needed to measure the R speaker (the secondary speaker connected to main KEF speaker) with 10.9 seconds sine wave sweep instead of default 5.5s one. Otherwise measured subwoofer SPL drops considerably (about 6dB). I don't think I have this issue before. The main speaker (and sub) can be measured correctly with 5.5 seconds sweep.
Something might be broken on my KEF speakers.

I tried decreasing the subwoofer volume from SVS app, unfortunately it didn't fix the problem. The SPL never increases with L+R signal regardless of the sub volume set on the app.

EDIT:
I've also attached those:

• L 75dB vs R 75dB measurement. R SPL under cross over freq drops with 5.5 sec sweep (but not with 10.9 seconds one).
• * L 75dB vs R 75dB (10.9s sweep) measurement. R SPL under cross over freq measures the same with 10.9 seconds sweep.

EDIT 2:
Looks like bigger KEF speakers like LS60W has a option named "Subwoofer Channel" on KEF connect app, which probably changes the way subwoofer signal is low-passed to multiple subwoofers. However LSX II LT only has single subwoofer output, not dual. The "Subwoofer Channel" option does not show up on the app (Unsupported on LSX series?).

 

[vroad]

I'll try sending my speakers to repair and see how it goes.
I was able to reproduce subwoofer volume fluctuation in other ways. Playing pink random noise limited to 30Hz - 80Hz range on REW and switching L/R/L+R channel (or even playing some music on another app) also causes subwoofer volume fluctuation.
That probably just means that my speakers are unfortunately faulty units.

Meanwhile I'll also try asking the same question on KEF subreddit, with the steps to reproduce the issue only with REW test tones, to see if anybody has the same issue.

 

[LSPhil]

Hello, can you please explain the difference between the first and second picture? The other question is, why do you need the L+R, or do you skip the center?