Steve, it seems like if you have the standard jumpers, you have many combinations to try. You really cannot hurt anything by experimenting 🙂
Not sure you really need to use the custom slot/jumpers. Might have worked for the previous owner, but may not be necessary for you. Perhaps a custom resistance jumper to attain 400 ohm? That is not offered via the standard jumpers. Again, Peter states a min. 470 resistance for the Sussarro.
Just guessing here.
Also may be worth calling Soundsmith to ask what capacitance would work best with the Sussarro, as they don’t give that info (N/A).