The preamp is non-adjustable and so I want the best match for my system as it is.FWIW, I owned a Wright Sound WPP200C phono preamp and used loading plugs with it utilizing a RCA "T" adapters jacks. It worked exceptionally well, and I was able to use any MC cartridge I chose.
Wntrmute2 (Threads | Answers | This Thread)
If you're interested in more info, send me an email. I still have a few of the loading plugs in different resistor values, and I have the adapters. The plugs are easy to make once you see how they're assembled.
All are available for sale as I no longer own a turntable.