That because the vent releases pressure in the opposite phase. You are feeding the inverted phase signal into positive signal canceling portion of frequency, obviously the result is lower SPL and added distortion.It just reduced sub bass and added distortion, I think it needs an orifice between the driver and the tube also, to create An actual chamber. But yes, the simple tube vent results are interesting so I will do something with that. This was tested at a reduce volume, it is louder than indicated on the graph.
I understand you want concept similar to transmission line in speakers, but consider - bass frequencies are measured in meters, a few cm of tube does nothing to invert the phase of low frequency, you'd need at least 3-4m of tube to achieve the result you are looking for.