Complex sentence: The Marshalls used EL-34 output tubes, but as they could barely reach the rated output wattage Unicord replaced them with KT-88 tubes before shipping to U.S. vendors.
Split into simpler sentences: The Marshalls used EL-34 output tubes, but those could barely reach the rated output wattage. Therefore, Unicord would replace them with KT-88 tubes before shipping to U.S. vendors.