Just trying to figure all of this out. One site says figure your tuning dependant on port length, others say by width and height of port (square inches of port area). So what's the verdict? I have 2 SSA Xcon 18's wanting them in a 6.5 cuft (each) box tuned at 31 hz. Also, what is the difference sound quality and SPL wise between having one large port or a seperate port for each sub?

It's both. Port area and length determine the tuning frequency of a given box.

As for 1 vs. 2 ports, there is no difference. But 2 ports generally takes up a fraction more room for the same tuning frequency (port area the same between the 1 and 2 port setups)

2 ports also have more turbulence as well

6.5 for both 18s or each... if for both that is a bit small.. i would recomend atlest 12cubes

and port area is hight x width after that is determined you can determine your port length depending on your tunning box area and what not

I have used the port length calculator on the12volt.com and put in my dimensions for the box, with my port height and width and it gave me my port length. so does anyone know if that figures in the port displacement also from the overall volume of the box? I am looking to hit at least all 4 tones in Jeezys PUT ON, and maybe a little bit higher frequencies. Want good daily driver with a good competition system for the weekends. Putting an Audioque 3500d on the 2 18s.