dude_500: the blue line is definately not sealed, there must be some leak, a large one!
I'm using the same film as you were (2um) and with two dust covers my bass frequency response is flat just like yours with 6um (even flatter). Just look at the SR-009 measurements: free-air resonance is ~170Hz but when sealed the frequency response is flat (except the ~40Hz step).
Here's my response (2um diaphragm, two dust covers, driver in wooden earcup, leatherette earpads, little bit of damping on the back side of driver, bias 580V), finally with calibrated mic:
As you can see the bass is great even at 20Hz, so I think you must have some huge leak somewhere.