Martin is a great resource, and far more knowledgeable about these things than me. My 2 cents on what he suggested is as follows:
increasing the port length to 6 inches will flatten the response below 250 HZ, but will attenuate it dramatically. The filter will basically then attentuate the response for the HZ over that. The result:
a flatter response, but also MUCH reduced sensitivity. IF you are running tube amps, with low watts, this may not be a good idea. If you are using solid state, and power is not an issue, then you can try it. However, to me, sensitivity means more than just low watts, it means greater dynamics and more presence as well.

Also, I personally feel that adding electronics to the speaker is somewhat at odds with why we are sticking to a single driver design in the first place....i mean why not just go with a multi driver setup?

Why don;t you wait till we heaar it, and then decide? Again, i think too many of us get caught up in the frequency extension game, and miss out on why we have single driver setups to begin with.
I left you a message, with my number. Call me when you can.
thanx
-akhilesh