I'll wager that at least half of those that find the bass from their mains 'quicker' do so because someeone told them they should.
The other half probably benefit from in room placement of the mains giving them better upper bass than wherever they put the sub.
The other reason one might have 'slow' bass from a sub is if the driver is marginal and has been optimized for bottom end performance at the sacrifice of upper end dynamics.
I have modeled a 10" driver where I could get good output down to 17Hz, but the trade off was that the driver started to roll off noticably above 80 Hz. Changing the tuning so that the low end roll off was at 25Hz restored the top end.
On the other hand, if you use a monster driver like the Exodos Mal-X 18 you have enough excursion for sub 15Hz performance, but have enough power handling and a well enough designed driver that you do not loose anything at the top end.
I think that this is why Axioms subs sound so good with music. Nothing is sacrificed in the upper bass to get more low end distortion.
If you are really curious about stuff like this, go over to avsforum.com and search the DIY speakers & subs forum for user Mark Seaton. He has made a number of excellent posts where he talks about sub desing and tradeoffs.
