acoustic neck warp

I bought a cheap acoustic 6 string (kona $120) at a pawn shop last week and when I got it home and put new strings on it I noticed a buzz at higher frets (14-17) mostly on the G and B strings. I have adjusted the truss rod out all the way and have done some sanding on the frets as a last resort but I am still not satisfied. I wanted to know if there was a way to get the truss rod out even farther (it is as loose as it will go) or if humidifying the neck and keeping pressure on it might make the tiny bend I need. Of course any other advice is welcome.