Sub DIY vs mass produced?

Is there anything that makes a mass produced sub better then something I could make myself? I want to add a pair of subs to my ht room & I would like to go with pair of 12's. I have a extra adcom gfa-5500 that I was going to use to power the speakers & possibly buy another gfa-5500. I was planning on using Image Dynamics IDQ12's for the speakers in a 4cu ft box tuned to about 20hz (that's what someone suggested I do).

IS there a reason a store bought mass produced sub would work better like a velodyne, JL or whatever?????

thanks