For me, the great advantage of subs is the ability to add EQ to the bass. The best speakers in the world may measure beautifully from 20 to 20K in an anechoic chamber, but you don't want to see how they meausre below 150hz in your room. Trust me, it ain't likely to look very pretty. While EQ may violate some audiophile golden rules, bass only EQ strikes me as a great way to have (most of) your cake and eat it, too.
Bob's comments about the bass content of music are generally true, which means that - IMHO, IMHO, IMHO, - your choice of sub is probably less critical than proper integration and EQ. I use a Velodyne SMS-1 sub controller ($500 street price) and it works great! You can also use an NHT x-over ($300) if you don't want the Velo's digital x-over in your main signal path. If you go that route, you can use the Velo for room analysis and EQ or you might prefer the new Audyssey bass EQ box from SVS for that task.
PS While sub choice may not be the most important factor in getting better deep bass, I'd note that the SVS subs do appear (from test results at AVTalk.com) to represent great value.