R-90 was manufactured around 1987 and was available in three configuations: records only, records/CD combo, records/video combo.
You likely have the 6-09931-02 130 Watt amplifier. The tone control slide switches (3 position) are located on the preamp board and accessible thru the amplifier front panel. This amplifier is well known for bad solder joints at the tone control switches, connector header pins, and at the voltage regulators on the preamp board.
The following capacitors go bad on the preamp board due to the heat of the voltage regulators:
C70 10 uf @ 50 volt
C63 100 uf @ 16 volt
C64 100 uf @ 16 volt
C65 100 uf @ 16 volt
Replace them and then see how it sounds.