Noobie Question about WHY to upgrade

I've had my home theater for almost 9 years so I'm not really a noob on HT, but I am when it comes to subs. While I'm on my third projector and have upgraded both my recvr and DVD (to BluRay) recently, I've kept the same speakers: MLs, which sound great and I have no interest in changing, and a relatively small, 10' 85 watt Velodyne sub.

I'm trying to understand the benefits of upgrading the sub. I was watching a movie last night and while I find the bass a bit boomy at times it does a nice job hitting lows. What confuses me is that if I calibrate my speakers, using something like Audessey or Avia, wouldn't I end up with the same end result on the sub? I would like a little more slam with the explosions, et al, but how do you get that if you're calibrating different subs to the same, presumably "correct", level?

And are the tonal qualities of subs that different? Certainly they are with mains, centers and surrounds; but having never really auditioned subs, I don't know.

I'm sure these sound like dumb questions but please bear with me. In particular, I'm trying to understand the relationship between calibration and higher SPL levels with 'better' subs. Do you just naturally run more powerful subs a little hot?

thanks!