To quote John Atkinson from Audiophile (October, 1990 p. 201-202): " Personally, I felt the older the '555's high frequencies to be somewhat emphasized, perhaps even grainy.
... the new '555 [has] a greater number of fuses between the rear-mounted, curved edge heatsinks, and the imput sockets are now much higher quality and use Teflon insulation. The power transformer has been increased in size, potted to improve heat transfer, and offers greater regulation. The output circuitry has also been changed, and the LF signal path has been stripped of polarized (electrolytic) capacitors. A new DC-servo circuit minimizes output DC offset.
Both amplifiers are polarity-correct, but the Mk.II version has slightly higher gain -- 27.75db vs 26.9db at 1kHz .... After three hours warmup, the heatsinks of the '555 II were appreciably hotter than that of the Mk.I, suggesting a higher degree of output device bias current. ... While I felt the older amplifier's high frequencies to be still a little grainy ... the soundstage was well-defined, the low frequencies weighty, and the music communicated well. Switching to the Mk.II '555 revealed an even deeper sounstage -- the accompanying French horn could be heard to be set further behind Mr. Minter -- while the voice was more robust in its midrange tonality. ... Via the original '555, the sound of the Steinway was a little lightweight, even "tinkly" in its upper registers, while the Mk.II '555 presented it with what I felt to be a more natural tonal quality. ...
Both amplifiers sounded pretty well equivalent in the bass with these kinds of music. ...
To sum up, I feel the GFA-Mk.II to be significantly more neutral as regards midrange and treble tonal qualities than the Mk.I, while preserving its virtues: excellent imaging specificity, a deep, well-defined bass, and a superb sense of dynamics."
... the new '555 [has] a greater number of fuses between the rear-mounted, curved edge heatsinks, and the imput sockets are now much higher quality and use Teflon insulation. The power transformer has been increased in size, potted to improve heat transfer, and offers greater regulation. The output circuitry has also been changed, and the LF signal path has been stripped of polarized (electrolytic) capacitors. A new DC-servo circuit minimizes output DC offset.
Both amplifiers are polarity-correct, but the Mk.II version has slightly higher gain -- 27.75db vs 26.9db at 1kHz .... After three hours warmup, the heatsinks of the '555 II were appreciably hotter than that of the Mk.I, suggesting a higher degree of output device bias current. ... While I felt the older amplifier's high frequencies to be still a little grainy ... the soundstage was well-defined, the low frequencies weighty, and the music communicated well. Switching to the Mk.II '555 revealed an even deeper sounstage -- the accompanying French horn could be heard to be set further behind Mr. Minter -- while the voice was more robust in its midrange tonality. ... Via the original '555, the sound of the Steinway was a little lightweight, even "tinkly" in its upper registers, while the Mk.II '555 presented it with what I felt to be a more natural tonal quality. ...
Both amplifiers sounded pretty well equivalent in the bass with these kinds of music. ...
To sum up, I feel the GFA-Mk.II to be significantly more neutral as regards midrange and treble tonal qualities than the Mk.I, while preserving its virtues: excellent imaging specificity, a deep, well-defined bass, and a superb sense of dynamics."