Necro thread, I know, but I thought I'd add something here just in case anyone with a C1 comes across this discussion and could use the information.
I had noise at my balanced outputs that was not present at the unbalanced outs, and I ended up replacing the SSM2142s with THAT1646s, with capacitors added in the locations that the 1646 datasheet indicates (important!) The balanced outs are perfect now.
Something worth noting is that in some versions of the C1 (earlier revisions, I believe) the SSM2142s are not present, and a small daughterboard with different components was utilized. In the schematic, it's just listed as an output module. (Both versions of the schematic are available here on GroupDIY.) It's possible that this setup could be converted to use the 1646s, since everything leading up to them is identical. It may or may not be worth doing - I don't know if there were shortcomings in the original output stage or if they switched to 2142s simply for cost reasons.
One more thing to note: I had to replace my LM13600 transconductance IC (the heart of the compressor section, really) due to bad distortion during compressor use. They're not available anymore, but I found someone selling NOS on ebay. The LM13700 was the successor to the 13600, and seems to be a bit cheaper when you find them for sale, but they're also obsolete and have to be sourced through private parties instead of the typical commercial suppliers. They're supposed to sound the same, but I wanted to be certain I was getting as close to the original function as possible. If you can only find the 13700, I wouldn't hesitate to use it.
The failed components in my C1 were the result of damage due to some bad PS caps and resultant AC ripple, so if anyone hasn't re-capped their PS I'd certainly recommend it as maintenance. These units are old enough that failing electrolytics are no surprise, and even more so because most gear uses 85C caps from the factory. Like many people, I didn't think about the damage potential to other components when a power supply goes wonky - I understand now. I just had to replace a voltage regulator in an SPX90 for the same reason.
