I am an EE with lots of experience with consumer electronics. I have to say, that your symptoms, if I understand them correctly are VERY strange. In the case of low power, the audio would be the first symptom. (distortion or lower volume). Adding a lamp etc to the load, would hardly make any difference in station reception. It would probably require a drop of 4 volts or more for it to bother the RF reception. If that were the case, you would certainly notice a drop in the lamp(s) brilliance. In any case, the first thing would be to check the voltage to the unit. With no other loads and then keep measuring as you add loads. As a matter of interest, does your FM and AM do the same thing? Any other devices that use this stero? CD's etc. I would try that.
Jim and Katy
w/ Shelby and Sealy (Bernese Mountain Dog and a Nice Mut)
HAM CALLSIGN - AI4SR
EE, IBMer, Race Car Builder, now Retired.