Sennheiser switched to a different earpad vendor some time ago; ASR measured the 600 series with new earpads, and they sound brighter than old pads like you mentioned. The way they can be told apart is that old pads have black mounting rings on the back and new ones have clear mounting rings.

The same applies to the 500 series, as seen in this video. There's still plenty of new-old-stock pads on the market (at least for the 500 series) so that might be the difference between the ones you have.