Help w/ Proton 300/301 Table Radio Glitch?

I recently purchased an NAD #515BEE CD player to use w/ a vintage Proton 300/301 table radio connected via standard RCA cables. In CD mode, it only plays at a loud volume and the Proton volume control doesn't change the volume, even when turned all the way down. In FM or AM mode, the Proton volume control works fine, and the NAD, likewise, works fine when attached to another vintage receiver. Any ideas?