I've been steadily sinking lower and lower for a couple of years now. Right now, the top of my thighs are about parallell to the floor when my heels are on the ground. I play heel up, however, so when I play, they actually point slightly upwards from my hips.
I've found that my groove and power improve when I sit lower. I do lose a little bit of foot speed, but I'm not much of a double bass player, and mostly use it for accents and fills, so that's not really a problem.
Another advantage of sitting lower for me is that I'm quite tall, so when I sat high I had problems sitting in on other people's kits, since they were almost always set up too low.