I just want to share my experience between Sennheiser HD660S vs Hifiman Sundara, particularly on imaging, instrument separation, and soundstage.

https://imgur.com/7QtQ9WW

Previously, I thought Soundstage is everything for realistic and enjoyable experience. Now I have experienced HD660S, I feel now that Imaging has a bigger effect on realism or immersive experience.

Here is how I define (or feel) these parameters:

• Imaging: this is the 3-dimensional mental positioning of the sound. This includes angle and distance. (Note that when I say imaging here, this is for music only and not absolute directionality for fps gaming, because neither of these 2 headphones are good for fps gaming).
• Instrument Separation: different sounds that occur at same time, can be heard as distinct instruments.
• Soundstage: size or distance where the sound is coming from.

HD660S is more intimate, as everything is in smaller scale. But in this smaller soundstage or environment, it actually produces more imaging information and instrument separation. I feel like I am on the stage with the band. Vocals are very forward. My mental image of vocals are typically less than 5 feet in front of me. Then instruments are just slightly further away, like 5 to 20 feet away. Even in this small soundstage, there are multiple layers to the sound. Some instruments sound closer, while others sound further away. As result, I feel more 3-dimensional imaging. And when different instruments feel like they are at different distances, this also adds to instrument separation.

Sundara has larger soundstage for sure. I feel like I'm in the 1st few rows in the audience listening to the band on stage. Everything feels bigger. Vocals are more distant. And then the instruments are even further away. Sundara has good instrument separation. I can clearly hear each instrument distinctly. However, Sundara feels 2-dimensional with instruments, in that they are all at similar distances and not layered.

As with many mid-fi headphones, Sundara does a good job. The 3-dimensional feel comes from combining vocals and instruments. Then HD660S takes it to another level, by adding layers of information for different instruments.

What is more important, more enjoyable, or more realistic? Probably depends on the music you listen to and your preference. Before, I thought soundstage is more important to provide that immersive experience. For sure, having soundstage vs no soundstage is the first step. But now I have heard HD660S, I feel the imaging and the layering (even within a smaller soundstage) actually results in more realistic and immersive experience.

HD660S was not price competitive previously, as it was $400-$500. But HD660S price has come down recently. I got it for $250 for refurbished from Sennheiser (but not always available). It is $300 new on Amazon, from 3rd party stores. Basically Sundara and HD660S are the same price now.

I bought the HD660S thinking it was something I can use for vocals. I wasn't expecting it be an all-around headphones. I was really surprised how well it performed technically, and how it actually outperformed Sundara. It kind of throws out my incorrect assumption that planar magnetic has better technical performance.