So if you want killer subs DIY is the way to go if you have the means to put them together and it will be cheaper. Probably can get 4 very nice 18" subs for the price of an SVS PB4000. Visit the DIY forum and you will see some bass enthusiast putting together some very nice stuff! Parts express 18" Ultimax is a favorite paired with Berhinger NX3000D or 6000D amps.
https://www.avsforum.com/forum/155-diy-speakers-subs/
https://www.avsforum.com/forum/155-diy-speakers-subs/