You are going to be hard pressed to really find a full replacement for the Amperior. In terms of portability, ruggedness, solid fit, with punchy detailed sound. It was Sennheiser move to make an audiophile version of its classic professional HD25. The HD25 actually has a more neutral sound signature than the amperior. Both are great.
I'd recommend maybe looking at the latest wireless headphones such as the Bose QC35 and MDR1000x. Sennheiser also has some offerings. While they may not give you a huge improvement in sound quality, they will give you the features that can come with wireless and active noise-cancelling. Both QC35 and the 1000x are tuned more for a friendly sound signature, and both roll off the highs a bit, but this is fine. They have larger ear cups than your Amperior and will sound different because of this. They also will heat up more in the sun.
If you really want portability, and small form factor, consider get a pair of IEMs.
