Or perhaps I could use one of the loading resistors on the input RCA's which might do the same thing.
No, you can't. But it depends. The reason i asked Joe to modify my JLTi phono stage with 500k Ohm internal resistor, so i can add any lover value in the dedicated pair of RCA plugs on the back. This is how it works if i want 100k Ohm:
The formula for parallel resistors is simple if you have a calculator.
R=(R1*R2)/(R1+R2)
If R1 is 500K and R2 is 100K, then multiply the two. You will get a large value.
Next step, add R1 and R2. You will get a much smaller value.
Now divide the larger value with the smaller value and you will get the final value.
In the above example, it is 500K x 100K = 50,000
And 500K + 100K = 600
Now divide 50,000 by 600 = 83K333 or 83.333K if you prefer.
If the value of the loading is way under 500K or 1M, then the value in the
loading plug will completely dominate.