@ChrisCables I have a HD545 too but measures a little differently to
@solderdude 's. It is dark blue rather than grey and has no rear foam damping as it had perished, but has a black paper screen like his.
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The rear grille does influence the frequency response which is unusual. I've seen a narrow dip at ~4kHz in some HD580 measurements which has a similar grille pattern. HD565 is probably better in this regard due to having a metal grille more like HD600/650.
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3rd party pads work ok but still ends up losing treble and bass. I actually don't mind the sound with the 3rd party pads and a bass boost.
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Rear damping brings down mid-bass as expected. Wouldn't recommend adding foam damping with 3rd party pads as the bass loss is already severe.
n.b. I realised afterwards I might be getting that 4kHz dip from the grille due to having no rear foam damping but haven't tested with foam + grille to be sure. Would explain why solderdude's doesn't demonstrate that effect.
The plastic headband is slightly longer and much less stiff than my HD6XX. It is quite stretched out so has much less pressure on my head. This may contribute to the lesser bass. Gives the psychological effect of sounding a bit more open I think due to less feeling of something on your head.