Unless they have a fundamental speaker misbalancing error that needs correcting, that sort of recalibration is going to disturb the balance - you're effectively reducing the relative level of all mono content, by reducing a speaker that's dedicated to mono. It's not just a speaker weighting change. To actually "even" it you'd need to send more mono content to L+R, not just reduce C.I just noticed that Dolby Surround puts a lot of signal to the center channel, more so than Auro3D. In order to level things out and make it sound even better, I reduced the center channel level by -6 and increased the overall master volume to compensate. I've noted that in my settings for anyone wishing to replicate:
I would be interested to know what the balance of L/C/R mono content looks like in Dolby Surround. I'm used to Dolby Pro Logic II, and I just spent a few minutes testing what it looks like.
Test signal was L+R pink periodic speaker cal (500-2000Hz) noise from REW. Measurements all done at (or close to) main listening position, isolating particular speakers.
(CW=0 is what Dolby Pro Logic II Movie does, and CW=3 is the default for Music mode, so L+R total level equals C level. That signal was at 78.8dB SPL on L+R without the Dolby Pro Logic II enabled, so we can see that CW=7 doesn't interfere with L+R at all - it totally mutes C. Whereas with CW=0 L+R still get a bit of leakage 30dB down).
In that test, it does look like C needs a bit of a boost - possibly 2dB. A separate test suggests that's half down to my calibration or mic position - C is 1dB low compared to L without the DPLII. The other 1dB could just be general uncertainty about how signals acoustically sum.
I had assumed the DSU "Centre spread" was something like Centre width 3. If you really needed to drop it 6dB to get in line, that would mean something more like Centre width 1. But then having done that, all your mono would be far too quiet. Unless they'd screwed up and it was something like L+R=75dB and C=81dB, so the total mono level was too high to start with?