[iulianm]

Since I moved from '19 Mazda3 hatch to '23 CX-30 out of curiosity I've measured today the CX-30's system and did some listening tests and I have some impressions and data to share:
First of all, I haven't found a similar photo for the CX-30 but this is the speaker placement on Mazda3 hatchback and from my (limited experience) is one of the best systems in it's class.

I am no expert in audio apart from amateur audiophile; I have a UMIK-1 calibrated microphone with which I've done the measurements.

Some impressions:

• To my ears, the system in CX-30 sounds a bit different compared with the one from Mazda3 and it could be due to different seat position and how speakers fit in the cabin
• like in the Mazda3, the system seems to be bass heavy (linear mode helps reduce a bit at lower volume as standard mode will pump the bass even more at lower volumes)
• there's a significant drop bellow 35-40Hz and the bass seems to be localizable as coming from the trunk; I'm not sure what is the crossover point but usually above 80Hz you can localize the source of the sound.
• big difference in sound quality between bluetooth and cable

This is the frequency response between the two cars.

Then I have done measurements at two different seat position and I haven't seen much of a difference (but there is one nevertheless):

Not much difference:

I have compared my measurements with what savagegeese (Youtuber) measured on Mazda3 hatch and the same drop under ~35-40Hz is observed on his measurements as well.

I also have a '21 BMW 3-Series with Harman Kardon system and some notes about it:

• The HK has 16 speakers and the subwoofers are under the front seats
• Bose is more bass heavy while BMW's is a bit more balanced
• NVH also helps the sound in the BMW
• at Mazda you can hear that the subwoofer is in the trunk while in BMW everything seems more balanced (the subwoofers, as I mentioned above, are under the front seats)

Here's the measurement between these two:

 

[iulianm]

I haven't experienced any rattles however the bass on Bose system falls down under 35Hz abruptly.
It could be that Mazda by design has dropped the frequency bellow 35Hz (maybe lower distorsion or prevent some rattles) and with an added sub that goes down to, let's say 20Hz, to get some rattles.

I suggest to get UMIK-1 and with the use of free software REW and a help of a 10 min YouTube tutorial you'll be able to do a before & after measurement and might help you to do some calibration (phase, level matching for example)