First think about it:
What happens if you do not use some muscles?
Even with healthy young men muscles get weaker in a very short time if they are not used.
Therefore all the chairs that provide more "comfort" for the person sitting on it will result in causing muscle degradation, back pain and possibly even more worse things (prolaps).
Best is to have body positions which keep some load on the muscles.
Some people think: "Ok, I'll sit for 6 hours in that comfortable chair and then I'll have 30 minutes workout."
But that's completely different (much worse) than having 6 hours small activity in the back muscles.
(Also: Might it be better to trade in a body position that would allow to sleep or in a position that requires the senses to be activated?)
To achieve this something like this item would do the job:
http://www.amazon.com/TKO-Anti-Burst-Fitness-Ball/dp/B000FH2W4A
(Drawback is it needs quite some space and doesn't look so good.)
This kind of chair would also be ok:
http://www.amazon.com/Boss-Beige-Caressoft-Medical-Stool/dp/B0019QGVJ8
Another solution is to put something like this on top of a simple chair:
http://www.amazon.com/GOGO-Balance-Exercise-Training-Stability/dp/B001KOD92Q
As often cheaper does not mean worse.
Sitting all the time should be avoided.
You need frequent changes of body position: Sitting, standing, moving around.
It's even evidence based knowledge that when already back pain is present it is better to keep the back moving instead of immobilizing it.