Since you have mentioned it a few times: What is a "high fidelity guitar amplifier"? I always thought these amps were about being "low fidelity" in a well-controlled way, such that one can dial in harmonic distortion, saturation and clipping to taste?
Edit: You may use my freshly forged brand claims "High End -- Low Fidelity" or "High Low Fidelity" free of charge. 
A Guitar Amplifier is an amplifier and speaker unit designed for guitarists.
It has EQ, gain, volume controls and a speaker that are designed to introduce distortion.
This allows a guitarist to add colour to their audio path and have high sound volume (Jimi Hendrix style).
A High Fidelity Guitar Amplifier doesn't introduce this distortion into the audio path.
A guitarist would introduce all the distortion effects given by a Guitar Amplifier (Marshall style) before it enters a High Fidelity Guitar Amplifier.
So, a Guitar Amplifier uses a high power, high distortion amplifier to drive a high power, high distortion speaker.
To get higher fidelity, the two issues to address are the amplifier and speaker.
The amplifier can be any off the shelf, high fidelity, high power amplifier.
The speaker in a Guitar Amplifier is designed to run in an open enclosure, giving maximum sound power output for a given electrical power input, but at the expense of distortion.
The speaker in a High Fidelity Guitar Amplifier is designed to be mounted in a sealed enclosure, where the volume of the enclosure is dependant on the Thiele and Small parameters for that speaker.
This gives a maximally flat frequency response from the speaker, but also means the speaker delivers half the sound power output for a given electrical power input.
From the Oxford, 'fidelity':
the degree of exactness with which something is copied or reproduced.
"the 1949 recording provides reasonable fidelity"
