You can remove the tweeter and use that hole, b ut you will need a PVC or cardboard tube of say, 1 1/2 or 2" diameter. What I have done on my cabs is mount it in a slightly larger piece of plywood ( say 3" by 3") I then use a gasket and T-nuts to mount it over the hole that is big enough for the tube to pass through. This way you can experiment with different port lengths to find the best sound and response. If you just use the hole without he tube the enclosure would be tuned to too high of a frequency. With tuned-port cabs, anything below the tuning of the cab (determined by port length and diameter) would leave the speaker unloaded, meaning that the speaker sees any frequency under the tuning as free air. Which means that if he tuning of the cab is say, 60 hz, that the speaker would be behaving as if in free air on any frequencies under 60 hz. ( which means flopping around as if not in a cabinet) You'd be surprised how many bass cabs are tuned too high. (SWR, for instance) I typically tune my cabs low, so that a speaker would still be loaded for a B string. And before guys like Francis or amp manufacturers have come out with subsonic filters the subsonics from piezo pickups would play havoc with speakers. So with my cabs, the bass would have to be boosted a bit more, but because of the tuning the speaker would always be loaded. (which is why I would strongly reccomend one of Francis' high pass filters) It's all personal taste, soundwise as far as cab design. My point is that in the real world with access to eq there are many more practical aspects of cabinet and tuning design than frequency response and impedance curves.
Eric--I would recommend ANYTHING over an EV. My least favorite speaker. I would recommend a Beyma 15G40 or try to find a JBL K140. The K series is my favorite JBL by a long shot. Any of these 15's are going to have limited high frequency response. The Beyma is listed as 1500 hz, but I think that the high end of any 15 is going to be similar to the EV. Give me a call. I have a basement of blown 15's that you could have reconed. I have a few Gauss, an RCF which is very nice---but I just realized that I have a blown Altec 421a which is an awesome bass guitar speaker. A little lower power handling, but really nice bloom and timbre.