I upgraded from HE4XX to Sundara, I immediately noticed the brighter sound signature, better bass quality, and a sense of refinement not present on the HE4XX. However, there was something warm about the HE4XX I preferred, almost boxy sounding bass was fun with EDM, I was grateful for the Sundara, but It had less fun factor, ultimately I decided to keep Sundara due in part to build quality (Way better build) my HE4XX was falling apart. I think if you listen to a lot of dance music, EDM or hip hop, I’d keep the HE4XX.
Periapt cable is a great option, I use one with Ananda, it’s very durable.