I noticed that Rythmik Audio has several subs. For the same price you can get a sealed 15" sub, or a dual 15" driver or a ported 15" sub. They all have the same 600 watt amp driving them. What are the differences in between them? I'm not in the sub market right now, but just curious. It would seem that the dual 15" would be the best design, disregarding WAF.