Since the OPer said he doesn't have a sub so whatever the stock crossover frequency is for the door speakers could be in the range of 55 Hz and lower depending on the stock speakers.The subwoofer takes over the lower frequencies (30-80Hz) which let's the door speakers play the midbass (60-300Hz) cleaner without the low bass interfering. The stock WK2 audio system (without a subwoofer) uses oval 6x9 inch front door speakers to compensate for the lower bass frequencies lacking without the subwoofer. On the Premium Harman and Kardon WK2 audio system, the front door speakers are round 6.5 inch drivers. And the subwoofer covers the lower bass register.
Could be the resonant frequency of the door speakers (without a factory sub) were designed to permit frequencies down to 30 Hz?
Or maybe the stock speakers have both bass and a mid range cones then some of this might make a little more sense?
If thats true it doesn't explain the -8 db dip at 55Hz unless the stock crossover is exactly at that frequency which is odd considering no sub is involved and the OPer said it sounds good down to 30Hz.....was a good amount of low bass (30-40 Hz), but overall something did not sound right....
OPer i'd get a legit frequency sweep of the audio system and not some app before going down the rabbit hole with this.