Google "ttg test tone generator free" and download Timo Esser's tone generator that is free for a month. If your puter isn't connected to your system, just make some tone files and sweeps and burn them on a disk. Then you play a frequency from the generator and measure it with the SPL meter.
Be absolutely sure your other speakers are set to small or are off for running that kind of test or you'll blow them for sure.
Keep the port outside the box, so you can easily try different lengths first. Don't put it inside until you get to the tuning you want.
You might be able to get away with a matched pair at the front of the room in the corners with the right color scheme. I think that will load your room better, but that's probably just a few dbs at most.
Before building something new, check the SQ and max limits with the top cap on. Also with the top off and lower tuning.
Yes I think a smaller diameter port and/or longer port will help control excursion to a lower frequency, but too low tuning could cancel some output from the big chambered end because they are at a different phase.
Just for grins, next time you have a sheet of ply or mdf, before you cut on it, place it over the opening leading out of the room and have someone hold it in place while you play some loud LFE's to see how much more room gain is possible. That way you know whether or not it's worth trying to figure out if it's worth any trouble trying to make that work.
I'd say measure and listen to what you have first. Going for extreme output is kinda fun, but realistically it will never be used, because your volume would be up way too loud for the rest of the audio spectrum. Plus, if you do use it, you'd have to start worrying about cracks in sheetrock joints and other damage to your house. Also, Dad's setting the example now, you may not enjoy the 7yro's taste in music in a few years and he/she will want to blast their's too.
Keep in mind that HWK size is about 100L too small for true EBS tuning. It looks like Bassboy and I are going to try some new that we've been discussing over in the HT section. If my 10hz version works, using a 15" driver, then that might be your answer once you want to cut some wood. Tom Danely, a renown sub designer, is getting 120+db at 20hz with a pretty compact model with a 12" driver. Mine will be 8ft x 1.5ft x 3 or 4ft and it looks like you have room for that behind your seating area.
http://news.ecoustics.com/bbs/messages/10381/169045.html