What we're dealing with here is essentially a voltage divider with a reactive load that varies with frequency. If the output circuit is a constant voltage source, it isn't as sensitive to reactive loads as a constant current source. Some SET amps have high output impedance, which makes the amp act like a current source. As a result, load impedance fluxuations cause corresponding response fluxuations.

Here are some other good links:

• Highly Reactive Loads and SET Amplifiers
  
• Effects Of Source Impedance on Loudspeakers
  
• Variable Amplifier Impedance
  
• Feedback and fidelity
  
• What is Negative Feedback?