That measurement has bad seal; the bass should extend flat on these headphones. The pads are shaped to fit a human head without having to clamp much, which makes sealing on an artificial head problematic.
I own a pair of Lambdas, so I am familiar with the fit. I agree, it is easy for a poor fit on the test system to mess up the measurements. All I was doing was looking for some confirmation that one could end up with test results as poor as Amir is getting.
I guess we need to see Amir's SR-303 tested on this system to put a baseline under the amplifier tests. If the HPs on the test rig are not providing a representative FR, it would be good to know. No matter what, the tests currently have the HPs 20dB down at 20Hz. After that both amplifiers are just doing the best they can, and behaving pretty much exactly as one would expect.