How does a ARC SDA1 sound, like a 100.2?

The SDA1 circuit was basically the same as the D130, just a 4 channel version. The 100.2 was a newer design with fewer gain stages (just 2) and better transparency.

Generally, all ARC amps share a certain house sound. ARC solid state amps will more closely resemble ARC's own tube amps in certain ways, but neither they, nor any other solid state amps, ever capture the sheer openness of tubes. They also have yet to reveal tonal colors and textures the way that good tube amplifiers do.

The "unforgiving" nature of the solid state amps comes from the transistor's tendency to be a bit hard edged and grainy compared with the liquidity of tubes. ARC has choosen to make their solid state amps of recent years as revealing and transparent as possible, while other manufacturers instead choose to darken or soften the presentation to make the transistor amps less bright and less prone to cause listening fatigue. There is no right answer here, only each listener's preference.

I'm an ARC dealer.

I have owned various ARC products for about 20 years now, tube and solid-state, including a SDA-1 and a 100.2, and what I heard was extremely similar. Both present extremely life-like imaging and dynamics, characteristic of the best tube amps. These two characteristics add a sense of realism that has kept me a fan of ARC, particulaly their amps, for so many years. Neither amp is rolled-off or softenend, as many tube amps can be, which often provide a warm and smooth timbre to every instrument and every voice. This can be very pleasant with small combos of acoustic instruments, but not accurate for a wide variety of music listening. The bass is not like a tube amp either -- both amps are VERY tight, powerful, and extended. The only possible problem is that both of these amps are absolutely revealing and clean, with a tendency to be unforgiving. This makes choice of speakers and speaker wires critical, and still can leave you wondering what is wrong elsewhere in the system. Usually, you just cannot throw an ARC amp into an existing system and expect an instantaneous improvement. You will definately hear some improvements when you try either of these amps, but you will probably also discover problems that had been too subtle to notice before. My opinion is that all ARC equipment has a house-sound that transcends the controversy of solid-state versus tubes, and that can reveal problems with other components in the system.