Complex sentence: The influence of the speaker impedance is different between tube amplifiers and transistor amplifiers, principally because tube amplifiers normally use output transformers, and cannot use as much negative feedback due to phase problems in transformer circuits.
Split into simpler sentences: The influence of the speaker impedance is different between tube amplifiers and transistor amplifiers. The reason is because tube amplifiers normally use output transformers, and cannot use much negative feedback due to phase problems in transformer circuits.