What is the exact net internal volume of your box, taking into account the volume taken by the driver and the space taken by the pipe? With that info and a confirmation of the pipe length, I assume that there is an elbow in your duct, I would be able to calculate the approximate tuning frequency.
The high pass filter should be set at a frequency which is not to be lower than the box tuning frequency. Below the tuning frequency, the sub is no longer loaded and therefore unprotected from cone displacements in excess of its maximum safe limit (Xmax). I'm surprised that the Parts-Express rep did not talk to you about high pass filtering. Did he mention what would be the box tuning frequency after the modification?
As for myself, I have never used high pass filtering on any woofer that I used. I don't play music or movies at very high SPL, so the question never occurred in my mind.