Third on capping your line. Not a good plan. One problem you are running into, like others have said, is running modern equipment through very old tech. If you were simply running a turntable and wanted the old school thing, you would be golden. But you are trying to take all of the goodies of the modern, digital age and mate it to something never designed for that. I'm with the others, sell and move on. Or, play with the tone controls for now and see what you can get out of it. It's simply not worth the effort and money to try to "fix" the sound with cables etc. You'll run circles around yourself.