@JRS The rough problem of chuffing and air noise in general isn't in the bulk channel, it's at the edges, and specifically at the mouth where the pistonic volume interfaces with unmoving room air. Generally the best way of mitigating this (beyond using a bigger port area in the first place) is to round off your port ends, reducing the velocity as the cross-sectional area expands, and increasing the effective interface area. The horn-like flare of a Precision Port is the natural extension (pun somewhat intended) of this. The obvious downside is that this is additional space eaten out of cabinet volume that does not act as effective port; it's also more complex to make and takes up more exterior room on the cabinet face.
The Monoprice cutaways show the port is moderately rounded at both ends already.
Another real-world fix is to ensure any higher-frequency port noise is just not pointed at the listener, with the idea the HF noise is going to be radiated less. This doesn't work in the OP's case of playing test tones with one's face / mic right at the front of the sub, but in an actual room, you may mitigate this by just rotating the subwoofer. You don't want it pointing at too close a nearby wall / subwoofer or you're effectively just making a longer port.
Usually when you see designs do strange things with the bulk channel of the port it's to try to control port resonance, or to prevent other higher frequencies already in the cabinet from escaping out the port (generally not an issue in a sub.)