What size subs? I'm assuming those are 12"s because of the dimensions you listed. If so...that rear (sealed) chamber is probably way too big. Bring it down by a couple ft^3 and add that space to the front (vented chamber). That should allow you to shorten the port a bit as well.
Note that those things will increase efficiency within the bandpass. The box you modeled will work very well and will be flatter over a broader spectrum of frequencies (mainly on the upper end), but I assume that you didn't design a 30 ft^3 enclosure with four drivers for perfect SQ, lol.