B&C 18SW115 Enclosure

I would not go with those older sub designs from b&c. Definately the 18sw115 designs do not have sufficient port area.

As far as Perth as a city goes, D&B is about as low as it gets. The club install market is dominated by mediocre sounding D&B systems, the small-med dance music festival market is a mixture of D&B and FK1, to my knowledge none of the FK1 systems have anything bigger than the F218. There is us with an xtro punisher system and lastly a kitchen sink of other stuff like Nexo, Void, QSC, Yamaha etc which I don't see around much. 

So maybe my experience is very limited, but from what I have heard here, playing low is not the name of the game. 

Interesting, I downloaded the Family matters track as WAV from youtube and compared to my non-youtube-rip wav, the only difference I could see was something strange in the >15khz region in the youtube version, otherwise I got similar results to you.

I had a quad of BFM T30's some years ago which I ran quite a few hours with techno music, when I built the Keystones I experienced an increase in SPL compared to the T30's but I also felt sub bass was compromised to some extent. With the T30's I chose tracks in my sets to include tracks with as much sub bass as possible in another way than I did when I got the keystones. I heard the same reaction from other DJ's who was not used to subs digging that low that they tweaked their sets to the lower octaves than they were used to do. I'm a bit surprised by the frequency plot, makes me think something else might also be at play here.

I ran both of those tracks through Audacity and in each case the maximum bass levels are occurring 50Hz and above, with even 40Hz around 12dB down.

Do you think that youtube is perhaps filtering out the sub bass compared to the original tracks?

If you play this kind of techno I'd suggest something that's digging deeper than the keystone/th18: https://youtu.be/jM8KEfjLpI0, https://youtu.be/gbLXq-AbzKM

If you play lighter house/techno without the deeeep rolling basslines they will work awesomely.

I've got a pair of keystones loaded with 18sw115-8 and like to play the really deep techno, it works and plays brutally loud and good but I could easily use 10 Hz lower extension on quite a few of my tracks. 

To able to adequately match spl with your tops you definitely need two big tapped horns each side. I think othorn or Skram, if they work with the driver would be good or keystone/xocth18 if you are ok with the higher tuning.

Good luck!