@imhififan If that schematic is correct, that preamp is susceptible to RFI injected directly into its input via the tonearm cable. So the 30dB peak that can result from certain LOMC cartridges would likely cause the phono section to make distortion- which would likely cause it to sound bright.
If you look at the left channel input, you'll see that the input isn't directly inside the feedback network. So when the peak is active (which can be energized and set into oscillation by the cartridge signal) it can overload the input, creating a distortion similar to 'transient intermodulation distortion' that occurred with some high feedback amplifiers made about the same time.
Loading is probably the most practical solution; If the input were modified with RF protection (and stopping resistors in the circuit) it could be rendered immune. But most phono section designers didn't take the significance of putting an inductor in parallel with a capacitance... they figured if they had enough gain and proper EQ they were home free.
I would start with something a lot higher than just 10X (maybe start with 1000Ohms) since what you're up to is killing the RFI generated. The highest value that can kill it will likely also be the best sounding.
