Oh yeah, they're rough indeed and I wouldn't pay that price.
I'm just tossing out ideas, I have no plan. I was more curious about the round port vs slot. Always looking to understand this stuff more, so I was more curious if that design is essentially a 606 with round ports.
Dont let my Bias skew your bias to much.
As far as building and designing a enclosure. I find slot ports difficult to deal with. but as a whole function the same as a round port. there is equations or end correction formulas to reduce the pain in the ass error factors of slots. but most free download calculators dont include them or the ones they do are usually incorrect.
If you really get down to it. a port doesnt resonate or move and accurately produce a waveform or signal like a speaker does.
its just pressure that resonates at a set frequency.
If understating the basic way pressure will radiate. a circle. then a ideal port would be a circle. its somewhat that simple.
a triangle port , square port, rectangular port. all work the same. the size of the opening and length will achieve resonance just like a round port.
the efficiency of a square or triangle or rectangle is lower, because the circular pressure trying to leave it. will always eventually hit more friction points. when this becomes more or less audible is like any port. depends on size and the pressure velocity
for the most part the bigger concern in port design is how much sound pressure the speaker is producing, hence at resonance the port will also have so much pressure leaving it at a certain velocity.
if velocity is high then eventually you will get a audible distortion called " chuffing" usually if the port, or ports is too small.
when working with a driver like the EV or many other 15" below 100hz real world power handling before fart out is usually around 80 to 150 watts. and likewise the ports will only be dealing with so much pressure. so its rather easy to get away with small ports.
when dealing with 200, 300, 500 watts of actual sound pressure below 100 hz then yes port design becomes little more important. basically the port needs to be bigger so it doesnt chuff at high volume.
most these system probably wont use a slot port. many do. but if they insist on using a slot port. any designer that actually " gets it" will use a more flared design called a trapezoid port. this will increase efficiency, and lower the distortion and even leakage that a normal slot has at very very high velocity
in nutshell the slot port size of the 606 is fine for most drivers. likewise 2 round ports that are 3" to 4" openings is usually fine for common 150 to 200 watt drivers. Meaning the usual " 300" or " 400" watt rated cabinets. that only realistically do 80 to 150 watts before they fart out or reach high distortion below 100 hz.
something your experiencing now and im sure your aware of the spongy ness or fart out of a single EV driver. and like most here have recommenced you essentially double the overall cone area of the system. by using a second exact same driver mounted in the exact same enclosure. and greatly increase the overall sound pressure before distortion. pretty much double or 3dB, meaning same watts and roughly twice the sound pressure. which is what i highly recommend. another matching driver like you have. in a matching enclosure