Here's the method described for lowering your TA.....

http://www.poolforum.com/pf2/showthread.php?t=191

You do use muriatic, and the pH and TA both drop, but then you raise the pH again by aerating the water. When the pH comes back up, then you can add another dose of acid to drop the TA a little further, then aerate to bring pH back up.

Janet