Firstly, congrats on the new car! I've had my 18' Signature for just over 6 months now and so far it's been fantastic so far!
To answer your question, the low-frequency response is undoubtedly not great from this Bose system. The system as whole is really just a hair above average imo. The speakers in the front doors are in fact subwoofers. They are the only speakers in the vehicle responsible for reproducing low frequencies. There is a resonance peak where they do sound good @ ~60Hz (judging by ear), but as you go higher the sound quality just drops off and it becomes annoying vibrations instead of proper sound. The entire idea of putting subwoofers in the doors is just awful and entirely cost-related I'm sure. But, thankfully Mazda has FINALLY
realized/admitted this and is going with a new design for the 2019 Mazda3. I seriously hope this carries over to the rest of the vehicle line then too.
There's a few things you can do to help with the bass and clarity though:
- Easiest way to increase bass is to fade the audio to the back of the car a couple ticks. While this will lower the overall volume up front just a tad, the subs are seemingly unaffected and will audibly sound louder since they're up front. This will also diminish the front sound-stage effect a bit, but in return gives you a more surround effect which I personally tend to prefer.
- Use an app with a built in EQ. Spotify for example, while not providing the best EQ, still gives something and allows you to make more precise sound adjustments before the audio gets to the head unit (+ works with CarPlay).
- A slightly more involved measure that I've done for better mids & highs is to replace the front dash speakers. Personally I've replaced all 3x front speakers with FaitalPro 3FE22 (4 Ohm) speakers and the audio sounds much more clear and full now. Lots of threads about them over on DIYMA. Power spec wise they're drop in replacements, but you'll have to get a little creative with the wire connections and mounting method. I opted to make some custom 3D printed mounts, but you kind'a can get away without them.
- Lastly, the most generic method is a trunk sub for added bass, but I think without upgraded mids first this isn't a good idea.