The 450 is a sealed subwoofer, it doesn't have the ports of the 350 to create additional pressure. It's a design trade-off, usually sealed subs will have have a tighter and more articulate sound but at the sacrifice of quite a bit of output. A lot of ported subs can sound 'boomy' and 'buzzy', and suffer other types of distortion that sealed designs, by their nature, can avoid. However, a well-designed ported subwoofer shouldn't suffer from these problems anyway. Think of it as a quantity over quality trade off in subwoofer design.
With that being said, I wouldn't worry about the A5-350 having poor sound quality, the guys at ED know what they are doing, and it will sound great. Also look into subs from Outlaw Audio, HSU Research, Epik subwoofers and Rythmik, which are other internet direct subwoofer companies. Along with SVS and ED they make the very best subwoofers at their price level.