I am getting myself a Sitmatic chair, because I sit here for 10-12 hours a day, too. I went to an office furniture supply company and tried out a bunch, along with my co-worker. I forget what brand he got, but all of the chairs were too tall for me (I have short legs). The rep at the office supply place showed me Sitmatic and since their chairs are totally customizeable, I was able to get what I needed.
If you look at their Beta line options you’ll see they have “Air Lumbar” and “Visco Elastic Seat” which might be what you need. The same options come in the Alpha models but so far I haven’t figured out the difference between the models.
I haven’t gotten the chair yet, as I just placed the order. But I really liked being able to customize it.
Make sure you go through an office supply place. We were able to choose 2 models and have them delivered to our office (our homes, actually, since we work from home) and try them out for 2 weeks before we bought. That’s soooo worth it if you’re going to be spending $600+ on a chair.