Okay, I think it's really important to just put in some basics about herbal remedies.
First off, everyone is different and different herbs affect everyone in different ways. An herb that could work wonders for one person can easily have no effect on another. This does NOT mean that its just the placebo effect, it means that we all have different bodies, inside and out, and herbs are much more subtle than pills (which are generally synthetic, stronger versions of herbs). If you are sensitive to herbs, then you might need far less than recommended or if you are not sensitive you might need far more. Using herbs by following the label and guessing at what you should take CAN BE dangerous, because of the factors listed above (for example, an herb that makes one person calm could make another totally bounce off the walls) The best way to find the herbal remedy for you is to talk to a professional.
Second, the best way to take most herbs is to get them fresh. Herbs loose their potency after only a few months and roots only last a year or two, depending on the plant and the way it's stored. Tinctures can greatly increase the lasting power for years, but they can also change the potency depending on the method of tincturing. If you can only get your chosen herb in the form of a powder, then you could try to find some empty capsules (they usually sell them at co-ops and health food stores) and then you can take them that way.
Third, herbs and vitamins are not very well regulated, at least not in the US. Most of the medical community in the US (regardless of what other countries such as Germany are doing as far as studies PROVING what little old ladies in the sticks have known for centuries) think that herbal remedies are just silly urban legends and don't actually work. So they're not too concerned when the herbal supplement has been sitting in a warehouse for months and is mixed with who knows what before hitting the shelves. Also (I know this is true for vitamins and I assume it's similar for herbs) most of the bottles you see of varying prices contain the exact same ingredients. There may be three big plants that produce powdered kava kava and distribute it to hundreds of other companies who put it in capsules and bottles and try to sell their label, not their own product. The best way to know what you are getting and that it's not treated with chemicals or extremely old is to get it locally or at least do a bit of research if you have to buy online. One really great company that I use is Wise Woman Herbal, I think they're based in Oregon.
All this being said, I tried kava kava and it did nothing for me. When I started to see my naturopath she had me stop taking that (along with rescue remedy, 5htp and all the other stuff I'd been self medicating with) and outfitted me with two supplements I'd never heard of and a homeopathic remedy (that is actually chocolate, I found out about a year into taking it). When I was trying to self medicate, I was getting nowhere because I really don't know enough about the intricacies of herbal medicine. There is so much to know that you wont find on the label or in a lot of books for laymen, that is why I now recommend talking to a professional. Even your local herbalist might be more helpful than doing it yourself. After taking what my naturopath prescribed, I'm SOOO much better and happier. She is such an amazing healer and I wish all of you could just see her, she's the first doctor in my whole life that I have ever trusted.