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output impedance. Ideally, you want the output impedance of your headphones to be 8x the output impedance of the amp, otherwise the impedance mismatch tends to bloat the bass. Whether you think bloated bass is a deal breaker or not for a tube amp is up to you, ymmv.
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power delivery and/or gain. Low sensitivity planars need more current than dynamic headphones to reach similar volume & control. “High gain” can help alleviate this somewhat by boosting gain before the output stage, but even so without sufficient current the sound may be plenty loud yet still limp and uncontrolled.
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OTC (Output Transformer Coupled). You generally don’t want OTL (output transformerless) with planars. OTL tube amps typically have higher impedance and less power, so they usually don’t work well with most planars.
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