I thought about it, and i found out I could make the slot port longer (ending off 3 inches away from the side of the enclosure) and have it tuned to 37hz. Sassmaster, how would i go abouts calculating the tuning frequency if i lined my port with polyfill? Would i go by calculating the new "apparent" volume of the port? ( i was already going to line 3 sides of the box with maybe 1" of polyfill).

 

looking at those plots i'd go with the red for sure... otherwise you're giving up alot of the nice deep stuff... the yellow has a -3db around 34-35Hz... alot of hiphop today plays well below this.

Also you could gain a good bit going a bit larger on the enclosure by the looks of it... keep an eye on your excursion/powerhandling curve however... I don't know the actual sub or the amount of power you're planning on using.

As for polyfill in the port... I'd advise against it and there is no real way to calculate it... it isn't like in a box as you have a high velocity of air traveling through the port and the polyfill itself will restrict this pulling your tuning down on top of the "Idiabatic/Isothermal" properties of the polyfill, there is no way to calculate tuning accurately.
Also the fact that it restricts flow, means less output... and who wants that.

However making the box larger will allow you to make the port shorter for the same tuning frequency

 

Well put Ross. I havent touched any MTXs in a while but I found i liked them best in mid 30s or so maybe a tad higher if you're more interested in output than SQ, but I really doubt I'd ever go above 40hz for a daily use enclosure.