Musical Fidelity Phono preamp loading

I just bought a Musical Fidelity LX-LPS. They are cheap now and I was wanting to use it to experiment with cartridge loading. It comes with 5 different loading plugs that you put into rca jacks in the back. I think. Does anyone have experience with this unit? It comes with no information on how to use the supplied rca loading plugs. There are two black ones that are closed-back lugs. Instructions say the blue ringed ones are for typical MM carts, and the green ringed ones are for typical MC carts. That’s all the information on those. Then there are three sets of open backed rca lugs. The sheet says the white ones are 1K. Ohms I guess. The red ones are listed as 200 ohm. The yellow ones are 500 ohm. But these are open at the back and accept an rca plug. The circuit is not complete unless something gets plugged into them. Would the rca phono outputs get plugged into these? Or the black lugs?  So far the test MM and MC cartridges I’m using sound best without the black plugs inserted. They seem to attenuate output. That is my guess. I have no idea how to use the other lugs to see what they do.  I’ve contacted the manufacturer but they just said contact the dealer. I contacted music direct but not heard back. Given that this is the weekend I’m not expecting to hear from them. So if any of you can help I’d like to experiment with this unit this weekend. Maybe I’m the  only one on the planet that doesn’t just know how to use this thing, but I’m still surprised there is zero instructions with the unit. And no review or other online instructions I can find. Thanks for any tips on use.