I have two Stereo 70s as well as Adcom 5802s and Sonic Frontier amplifiers. Both Stereo 70s have been updated. One has the original stock circuit with all caps and resistors replaced with modern polypropylene caps and metal film resistors. The updated stock amp sounds very nice. Sonically it has a lush midrange with a deep soundstage. The midrange is sweet sounding, bass is OK but not tight and the highs are pleasant.
The Curcio designed mod has both boards (4 6DJ8s). Sonically the amp is excellent, if the 6DJ8s are in good shape. The amp has a very open, clean sound, a deep and wide sound stage and tighter bass than the stock unit. Sonically it's not sweet sounding but seems accurate.
The Adcom 5802s by comparison (bi-amped) are Nelson Pass designs (3 gain stages). The amps produce very tight bass, a wide and deep soundstage, very low noise, and a smooth and pleasant high end with excellent reproduction of spacial cues (when they exist).
The Sonic Frontiers sound similar to the Curcio design with a bit more punch likely due to the larger power transformer and output transformers.
The Curcio designed mod has both boards (4 6DJ8s). Sonically the amp is excellent, if the 6DJ8s are in good shape. The amp has a very open, clean sound, a deep and wide sound stage and tighter bass than the stock unit. Sonically it's not sweet sounding but seems accurate.
The Adcom 5802s by comparison (bi-amped) are Nelson Pass designs (3 gain stages). The amps produce very tight bass, a wide and deep soundstage, very low noise, and a smooth and pleasant high end with excellent reproduction of spacial cues (when they exist).
The Sonic Frontiers sound similar to the Curcio design with a bit more punch likely due to the larger power transformer and output transformers.