Ah you’re misinterpreting the graph. That’s not a “boost” it’s likely just where they set the crossover to send to the woofers. It’s actually not that fast of a drop below 45hz, it’s about 5db. At that amplitude in the graph, if you play a 45hz sine wave, then play a 40hz sine wave 5dB quieter than the 45hz one, the drop isn’t that noticeable. While the addition of a true subwoofer could get you down to 20hz staying relatively flat, most music isn’t mixed with anything below 32hz anyway.
I’m glad the bass port defect fix solved the issue for you though, after having a loaner that had the defect I know totally understand why some complained about the bass.
As far as your question about Bluetooth volume levels, any audio played over your phone via Bluetooth is also dependent on your PHONE’s volume. I’m pretty sure if you increase the volume on your phone then it will play louder in the car.