I have a KDC-BT278U single din radio in a 69 mustang that played perfectly with some old sony 4 way speakers for years. Recently it sounded like the tweaters were blown....basically like metal rattling. I bought some low end hertz 6x9's but that didn't change and to be honest many reviews had issues with them so I was able to get a better Hertz one's and basically did the same. i did notice that switching from the radio to USB MP3 or BT on the phone music sounded awesome. Very nice bass response! I did replace the antenna and power filters updating the 15yr old ones but that had no affect so I'm leaning to the only thing left which is the deck. As far as i know the firmware hasn't been updated and not sure that would phase it? Anyone got any ideas? I also put a rubber gasket to the speaker mounts hoping to eliminate vibration.