raul. i googled amplifier output impedance and got several hits. The info if over my understanding, but will try to work with the ideas. I grasp that quality of transformers is vital in tube design, especially when driving a devil of a speaker such as are the Seas Excel. The motor on these speakers causes havoc on most tube amps. I assume.
Lets see if this link loads:
http://www.transcendentsound.com/amplifier_output_impedance.htm
Lets see if this link loads:
http://www.transcendentsound.com/amplifier_output_impedance.htm