Preamp has too much gain

Very easy to fix, just need couple of high-quality resistor to construct a voltage divider.

Assuming your preamp has an input impedance of 100kOhm, then, to reduce the gain to 1/3, you can insert a 200kOhm resistor in series to the input RCA.

The gain will be reduced by 100k / (200k + 100k) = 0.33x, i.e. 1/3 in this example.

You can also use a high quality variable resistor, such as a Bournes pot then you can adjust the gain with a screw driver.

The electronics should be run at the highest possible gain, at a given supply voltage, where feasible. For example, if the line stage is designed for 20dB of gain, and you run it at 5dB, the THD will go up tremendously, and the circuit may even become unstable. The higher gain also gives you the best S/N ratio.

It's the right way to pad down the signal level, then to lower the gain. The noise in the system is typically 'White', so the less gain, the less useful dynamic range.