Do this: Take a small wall transformer used to power small DC devices and power it with the output end disconnected from the device. Measure voltage. If you are using a 12V wallwart for this experiment, you'll find that the output voltage is much higher than 12V, more like 15V. Your converter is the same, an unregulated power supply.
Idiot proofing: Buy a different converter. All new converters will work along with any future solar installation. Or, rewire 12V circuits so that distribution comes from the battery and not a common connection close to the converter.
Ground your stereo anywhere a good ground connection can be established (I've always chosen a metal part with good continuity to the frame).
Are all alarms and standby circuit boards isolated when using that disconnect switch? Confirm by measuring draw at the battery terminals.
