Are there any studies about this, or why do you believe this to be true? Honestly, i also thought it was just a case of "the good old days", but after i tried kava i had from this time again, i noticed a huge difference. I took only 20-30g and it felt like 60g. Or even stronger.
We have been drinking and importing kava since 2011 and I ve been looking at lab data for years. And we do sell our kava through a kava bar in Auckland that gives us a good opportunity for continuous flow of feedback.
I just dont see any decrease in quantitative indicators for the same grade products. If anything, improvements in drying, packing etc seem to improve subjective perceptions of potency despite the same kl %. I also dont see why kava should become weaker if one maintains the same quality. If you buy mature and healthy plants, there is no reason why it should suddenly become weaker.
It is possible some kava from years ago had higher kl% bc it was pure laterals or it was unpeeled (skin has higher kl%). We do sell 100% laterals, but despite it being the strongest product in terms of kl%, it is very far from being our bestseller. People strongly prefer natural ratio single cultivars.
Your perception could be due to lateral content, or maybe just a subjective thing due to a different cultivar. People often report feelimg stronger effects when they switch between cultivars