The lamps are incandescent, they have no polarity and run on AC. You replaced them with LED's that are polarized and would typically require a current limiting resistor at a minimum. LED's are still diodes and if you place a diode across your AC line in the wrong way that's also running to the tuner I am not sure what would happen, especially if you did not limit the current. The LED's may have nothing to do with the tuner issue.
What is the part number of the LED you used and can you show a picture of how you wired them up?
On my receiver I created a small circuit to rectify the AC to DC in order to drive the LED's. This prevents the LED's from flickering. The circuit consists of some small diodes, a capacitor and a resistor to limit the current. I have the receiver apart so I could look and give you more details on what I used.
I am sure that I used this and the note on the bag shows what the rectified voltage would be. The 6 LED's are all wired in series with a 120 ohm resistor.