I've tried 2 different RF converters with the stock stereo and the CD sounds worse than tape or the radio

I'm not sure what's causing it -I have another car and while both cars sound similar with radio and tape, my other car sounds 100 times better with the RF converter

-any ideas?

On a different note, what's the pioneer 1.5 DIN stereo like -I assume if that was installed, the stock sub woofer system would work as it's attached to the speaker connectors?

thanks