should I retro-fit some Radio Shack RCA's to the other end and play with the level?

No. The line level converter is only for speaker-level inputs, not the same as the Sirius output.

I don't know why the sirius unit is causing distortion, it shouldn't. Can you describe the distortion? Perhaps it's not related to the RCA voltage level.

To step it down manually, you can buy a radio shack stereo potentiometer and wire that to some male/female RCA sets. Once I had an amplifier which didn't have an adjustable input level, and this is what I used to step it down. Worked fine.