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Oh man, I got some data. I'll try not to make a giant post here. So at this point I know the amps and drivers are fine. I matched the amp output voltage and disabled EQ. Lets start with some 1/12th octave RTA pink noise measurements. Mic at driver head position and was pointing straight forward (dayton ecc6). Just playing the individual drivers with xover at 50hz and 175hz (4th order i think) everything else is disabled. 5 - teal is left driver, 6 -gold is right driver. Have some pretty big nulls at 75 and 150hz when playing the left midbass. No idea what the 35hz peak is, cabin mode or something. When I measured with the sub ON it doesn't show up. [I reviewed some backround measurements, this is noise from the car running at idle] Near field measurements. I believe the settings I used were 30 Hz to 200 Hz over 5 seconds. I think we can ignore the level difference, I might have changed the output level to avoid clipping the microphone. I didn't take time to measure out the distance or anything. Just got the microphone a few inches from the cone and measured. pink - 9 is left, blue - 10 is right. This measurement was taken with the doors open as well. I repeated the near field measurements with the doors closed. I thought that the latched door might constrain the doors differently, green 12 is right, dark blue 13 is left. Im not sure what to make of this. Maybe I should have shortened the sweep length.. I any case, I think we can see that the midbass drivers are performing fine. Next I moved the mic around the cabin a bit and measured some additional 1/12th octave rta pink noise responses. Only left midbass is playing. Dark blue/green - 15 is measured 2 ft forward of the driver (just above the steering wheel almost). Gold - 16 is measured at the front passenger head. Green - 17 measured between front seat at head level. Purple - 18 Driver position baseline measurement. plots with 1/12 oct smoothing. Easier to read IMO So I'm starting to get the idea that this is some kind of cabin mode or some structure in the cabin is creating this null. Perhaps something in/under the dash. Here is one more plot that might demonstrate this the most. These are all measurements with only the left driver playing. purple - 18 measured at driver head. Red - 21 measured at driver head and right door is open. Green - 22 measured at driver head with all doors closed and Left driver window rolled down. I think its very interesting that null at 75 hz is complete missing with the Rt door open. Rolling down the driver window didn't help too much but I think the window glass is very close to the rear of the driver so perhaps that is not helping and this is a bad measurement. Going forward, I think I will try filling some cavities with some denim insulation material I have lying around. Open to any suggestions...