From what you're saying, it does sound like there is something in your substrate thats buffering your ph and kh, and preventing you from lowering it permanently. I doubt you'll have much success with anything unless you change your substrate. You need something thats totally non-reactive, thats not gonna react with your tank water and keep raising your ph. Have you considered pool filter sand? I'm pretty sure thats a non-reactive type and it can look really good as a substrate.