Maybe its a noise issue that the cap is hitting. I found bypassing a g2 MOSFET driven reference string with a 1uf cap cleaned up noise a lot. Just a thought and your results will be most interesting.
My versions of the SLCF have all moved towards a having a direct coupled front end triode for a bit of gain. This places the grid of the CF at anything from +90 - 180V, which eliminates the need for a negative rail altogether. I have built versions with a triode CCS at the bottom and versions with a pentode CCS, there is very little in it in terms of sound quality.
Shoog
My versions of the SLCF have all moved towards a having a direct coupled front end triode for a bit of gain. This places the grid of the CF at anything from +90 - 180V, which eliminates the need for a negative rail altogether. I have built versions with a triode CCS at the bottom and versions with a pentode CCS, there is very little in it in terms of sound quality.
Shoog