This was the next model. I haven't tried them but look pretty similar. Also out of production though but probably a bit easier to find second hand. https://www.amazon.co.uk/Sennheiser-Wireless-Bluetooth-Headphones-Compatibility-Black/dp/B007K3SB6K
Be aware that they had wired and wireless versions of these so you need to be careful which you are getting.
I don't think they have a modern equivalent.
Everything seems to be pointing toward the AKG or the Marshall. Everything else is too big and clunky for the gym. Far too many reviews on Amazon to find anything useful!
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This was the next model. I haven't tried them but look pretty similar. Also out of production though but probably a bit easier to find second hand.
https://www.amazon.co.uk/Sennheiser-Wireless-Bluetooth-Headphones-Compatibility-Black/dp/B007K3SB6K
Be aware that they had wired and wireless versions of these so you need to be careful which you are getting.
I don't think they have a modern equivalent.