• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Why would an amp designer intentionally use different OPT impedances for Left vs. Right?

Here is another amp with dissimilar OPTs, Pioneer SM-Q140. You can see one is slightly smaller than the other.

I think there was a time when there was a different view of the potential take-up of Stereo. The Pioneer was designed to be used when the only stereo radio broadcasts were one channel on AM, and the other on FM, and in those days most people had mostly mono recordings.

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Reactions: Robert H. Gribnau
If designing an amplifier today, you might want it if you want to bi-amp your speakers. You'd have to have two such monoblocks. Each of the monoblocks would have one output for the LF speaker and a different one for the HF speaker. You could also tailor the output transformers and frequency responses of each to half of the monoblock so each half only has to work over its intended frequency range.

kind regards
Marek