The service manual describes IOC as follows:
"An Input/Output Comparator LED is provided for each channel. Illumination may occur at such times as when there is too high an input level, improper load impedance, output stage clipping or even an amplifier based problem."
The green Signal Presence Indicator, on the other hand, is described as illuminating at any time the output voltage reaches 1 volt peak-peak or above.
Assuming both circuits are operational, the fact that the left channel green LED signal indicator isn't illuminating should rule out some of the circumstances that cause the red IOC LED to do so. Two causes that one would expect to be automatically ruled out are too high an input level, and output stage clipping. One wouldn't expect the latter when driving tweeters only, however the former could cause the latter.
Regardless, before assuming
'an amplifier based problem', suggest you attempt to rule out upstream or downstream external influences such as
'too high an input level' or
'improper load impedance'.
For example, you could try reversing the L/R signal input cables and note whether the IOC light still illuminates on the left channel, or switches illumination to the right channel. If the latter, then perhaps there is something going on with the crossover or pre-amp. Should the left channel LED still illuminate, another possibility that comes to mind wrt input levels as possible trigger is the front panel input level controls may be in need of exercise/cleaning. Perhaps, any contact resistance related distortions here may cause the circuit to trigger a response.
Conversely, if reversing the input connections L/R leaves the left channel red LED illuminating, try changine the speaker cable connections round back of the amp L/R to determine if the illumination changes channels, if so, then a load impedance issue may be indicated (stray speaker wire shorting? wonky tweeter?).
Should the left channel red LED continues to light, no matter the external input/output lead changes, then the amplifier itself likely has a non-linearity issue, as the circuit is later described in more detail as being a window on the amps feedback circuit...
"The front-panel IOC display is actually a window comparator circuit using two operational amplifiers (IC3C,D) and an LED indicator (E102).
Any small nonlinearity in the amplifier causes an error in the feedback loop to appear at the inverted input of the main IC op amp (U100). This means the main IC output (pin 6) will rise above its normal value in an attempt to correct the problem. This signal is then responsible for raising the bias voltage on IC3 and in turn activating Q114 which illuminates E102."