I don't know what you are doing in Hornresp, but for the frequencies that you are interested in, that port is definitely not "far away" from the driver. It's less than 1/4 wavelength at the port resonance, which is what will give you trouble if anything. Depending on how strong it is, you might want to try to knock down the port resonance with a PEQ.
that's what I thought (that the port is not far away in wavelength terms) but it seems like that setting has a relatively strong effect (in terms of making the port dominate output if you move the port closer to the listener). This makes me think that that setting doesn't do what I think it does (wouldn't be the first time in hornresp), It's described pretty plainly as
The distance from the port outlet to the listener can be adjusted if necessary using the length difference parameter. A positive value for path length difference increases the listener distance.
The path length difference is the distance from the port outlet to the listener minus the distance from the direct radiator diaphragm to the listener. The difference is positive when the distance from the port outlet to the listener is greater than the distance from the direct radiator diaphragm to the listener.
Example of the effect - http://imgur.com/s3IwddN