Hi Dean-
Although I don't feel like an expert on many of the things that pass through the forums, I've go t to tell you that as a guy who suffers from three degenerative discs and two bulgers, I know chairs.
The Herman Millers are awesome, but if you don't want to spend 8-bills, I understand completely. Stay away from the office stores, and consult the back-pain stores. I'm not sure where you live, but most metro-areas have back pain stores. Here in Colorado, we have Better Back Stores. A good place to start is Spine Universe (spineuniverse.com). They have links to different retailers. The retailer here has at least 30 different models of chairs from $200 - $2000.
Personally, I have a great wheeled office chair that was $399 (worth every penny), and one of those kneeling chairs that is also on wheels. The kneeling chair is awesome fro recording guitar and mixing and tweaking all at the same time. It also promotes playing with the guitar on a strap as when standing. My back is so thankful for this. I used to always sit in traditional chairs and play sitting down hunched over. This was bad for my back, and not the best for mastering the instrument for live use.
So, again, I suggest going to one of the back stores. They should have a superior high-grade inventory to try. Also, ours allows you to take a chair and try it for a few weeks, allowing for returns when a chair is not sufficient.
Good luck, and remember that sitting in your chair will have long-term influence on your body in ways that you won't realize when you're tracking or mixing your next hit.
Good luck with your purchase.