The Studio Projects C1 is a nice heavy, well designed Tansformerless LDC microphone. It has a combination pad and filter switch (Similar to the Rode NT2) which makes it even more versatile. The C1 and C3 circuits are built with fairly good components.
So what’s the problem? Like many transformerless mics, SP used a K67-style capsule paired with a linear schoeps type circuit. The result is an over-hyped top end and lots of sibilance. This can be seen on their own published frequency response graph:
The simple solution for this mic is to swap in a flatter K47-style capsule. The converted mic will have a smoother, fuller tone and still have great low noise performance of the C1 circuit. The Pad and filter switches still work as before.