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i had that same question. class g uses two voltage rails while class h changes the voltage in the rails. i could see class g being called two tier because of the two rails but i don't know why class h would be called that. and by the way, they are ab amps until they need the extra power. and no they don't have the distortion issues of class d. i've done homework and own class a class ab class d and class h amps. the only one i am not too happy with is the class d. i have powered pa speakers with ab for the mids and highs and class d for the bass. those sound good.