I have a Montgomery Ward box I'm trying to add a line level input to. I tried the cassette input but had really distorted sound. Next, I tried the external Mic inputs and it sounded much better but not perfect. The problem is that the sound level is pretty low compared to FM Stereo. With FM, the recording monitor LEDs always reach to nearly full signal while my line level source on the mic inputs never gets above about 1/3 of the scale and sounds much quieter.

After much research I'm thinking I have an impedance mismatch but I don't know anything about the mic inputs on these old boxes. It has dual 3.5mm mono jacks on the back of the case. Any idea what those would be?

My source is putting out 0.5v @ 10k ohms.

Any tips on getting full output from this?