why design angle into sub enclosure?

my friend said that when you design a (sealed) enclosure, it is better to make a paralellogram type box (where either the back piece of MDF or mounting piece of MDF is at an angle) rather than a square box (all 90 degree angles) because it takes a little bit longer for the sine waves coming off the back of the subs to reflect back at an angle to the back of the subs, thus creating some disonance because the sub will be producing a diff freq by the time it reflects back(considering it changed freqs). why is this good?

what angle is most preferable? rather, which will create optimum performance?

also, if anybody has designed a good box for two 12" Infinity Kappa Perfect DVQs, please let me know.

or if you're super cool and want to design a simple box for me, please do! mounting depth = 7" (178mm). cut-out diameter = 10-7/8" (277mm). compliance volume Vas = 3.31 ft3 (94.28 L). and [i dont know if you need this but just in case] free air resonance Fs = 22.01Hz. space is not an issue.
 
a standing wave is a wave formed within an enclosure (and maybe folded between 2 sides) that can be exxagerated wrt other frequencies.

Regarding the non-parallel walls, the reason is related to standing waves, . but has more to do with ensuring that the waves reflected off the rear baffle doesn't bounce back and possibly interfere with the wave emnating from the driver rear, possibly leading to
1.cancellations / exxagerations
2. depending on the cone material, if the reflected wave is strong, it can pass back through the (relatively thin) cone material further smearing the front wave due to its phase / delay difference

hope that helps