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A simple optical to coax conversion is more of an analog conversion. Without a doubt it could be poor quality.
I had to have a longer think about what you had said. The DAC in the amp is receiving a digital optical signal from another component that's 6m away. If you only have optical out and optical in to play with then a converter doesn't change the fact that the dac in the amp is doing the work.
Example of what I meant.
In this case it's converting to AES which is compatible with coax signals. Things do get a tiny bit complicated with cable ohms, digital should be 110ohm normally.
The main reason is to use the good DAC in the amp rather than a cheaper one in a converter.