How often should one change/replace their drum heads? I have had my single-ply stock batter drum heads for a good two years now, and although they sound great, they are very "denty". I bought some Coated Remo Emperors so I could have an older 60's sound, but I don't want to be wasteful and put them on when mine sound good already. I figured I should just put them on when I bust a hole in a head?