The Battle of the Soundstage – Audeze iSine 20 vs Sennheiser HD 660 S Comparison Review

Aueze iSine 20 vs Sennheiser HD660S Best Audiophile Earphones

A MajorHiFi reader recently reached out to us wondering how the Audeze iSine 20’s sound compared to the Sennheiser HD660S’s sound. I had never listened to the two side-by-side and was super inspired and curious by the question. Can the Audeze iSine 20 stand up to the big soundstage of the over-ear HD660S? Are the two completely different beasts, or are they comparable at all? Today we’ll find out with this Audeze iSine 20 vs Sennheiser HD 660 S comparison review.

The Battle of the Soundstage – Audeze iSine 20 vs Sennheiser HD 660 S Comparison Review

In the Box

Audeze iSine 20 Sennheiser HD660S
Audeze iSine 20 earphones  Sennheiser HD660S headphones
Detachable audio cable (1.5 m, 1/8” stereo plug input to 2-pin output) Detachable cable (with 6.3 mm connector)
Cipher cable (lightning input to 2-pin output) Detachable cable (with 3.5 mm connector)
Ear hooks 6.3 mm to 3.5 mm adaptor
Extra eartips Documentation
Carrying case


Audeze iSine 20 Sennheiser HD660S
Frequency Response 10 Hz – 50 kHz 10 Hz – 41 kHz
Impedence 24 ohms 150 ohms
THD <.1% @ 100dB <.04% @ 100 dB
Transducer planar magnetic dynamic
Type semi-open, in-ear open-back, over-ear
Price $599.00 $499.95



Because the Sennheiser HD660S headphones are over-ear and the Audeze iSine 20 earphones are in-ear, they obviously have some pretty drastic differences in their designs. The HD660S naturally feels more secure on the head, although the iSine 20 comes with ear hooks which makes them more secure. The ear hooks do get uncomfortable after a while, at least on my ears, and so they aren’t ideal for long listening sessions. In the same vein, the HD660S squeezes the head a bit but has comfortable cushions to soften the blow.


Both the Audeze iSine 20 and the Sennheiser HD660s are very well made and built to last. To my eye, the most fragile part of both are their cables. However, the cables are both detachable and so are replaceable if needed. 

Uses: Portable vs. Home

Both the Sennheiser HD660S and the Audeze iSine 20 were made with different uses in mind. At first I was going to name this section of the review, “Portability”. However, after thinking it through, it is worth acknowledging that both pieces were made with different goals in mind. The Audeze iSine 20 is super portable. Compared to the HD660S, it is much smaller, doesn’t leak as much sound, and doesn’t require extra amplification. It’s cable is also much more manageable. The HD660S is better for more controlled listening environments like listening at home or in the studio. They’re more comfortable for longer listening sessions and work best with a dedicated headphone amplifier.

Audeze iSine 20 vs Sennheiser HD660S Cables


Low Frequencies

Listening side-by-side, I was actually surprised by how similar their low ends were. Both the Sennheiser HD660S and the Audeze iSine 20 had clear, quick, and punchy low ends. The iSine 20 seemed to have a little bit extra sub frequency energy, so the lowest parts of kick drums, bass guitars, bass synths, and cellos seemed to extend a bit. Especially while listening at louder levels, these types of low frequency instruments had more drastic emotional impact than the HD660S. The HD660S on the other hand had a more neutral relationship to its low frequencies. They didn’t seemed to be emphasized at all, but still were audible and clear. As a result, they were a little bit less hyped, but more analytical in nature.

Mid Frequencies

The low frequencies of the Audeze iSine 20 had a more drastic sense of separation from their low-mid frequencies. Perhaps this is because the low-mids and the middle-mids felt a little bit scooped. However, its high-mids were emphasized and as a result, vocals sat higher in the mix than they did in the Sennheiser HD660S. Pianos, guitars, synths, and strings all benefited from the sub frequency boost. However, they felt less full and thick than the HD660S. The mid-frequencies of the HD660S felt pretty flat and, depending on the mix, had good separation between instruments.  Electric guitars and synths especially sounded whole and life-like. In general, the mids sounded a bit faster to my ears on the iSine 20. Its emphasis on the high mids also provided a clearer attack, especially for percussive instruments.

High Frequencies  

Both the Sennheiser HD660S and the Audeze iSine 20 had wonderful high frequency extension. The Audeze iSine 20 is a bit more accented up in the highs so it feels like it has extra high frequency detail and space.  It is also pretty warm sounding and rarely gets harsh with sibilance. I especially was impressed by the iSine 20’s depiction of jazzy snare brushes. In comparison, the Sennheiser HD660S is more neutral and, again, less hyped.


During my listening test I was most curious to see how the soundstage would differ between the Sennheiser HD660S and the Audeze iSine 20. The Sennheiser HD660S seemed to have a greater sense of depth particularly in the low end and midrange. Guitars and low frequency drums felt deep and spacious. The Audeze iSine 20, however, felt more deep and spacious in the high frequencies. As a result, the iSine 20 also had a taller sense of height. The sense of width between the HD660S and the iSine 20 was quite different sounding. The iSine 20 was much wider, but the HD660S sounded more natural, more intimate almost. It’s balance from left to right was detailed but still felt like a gradient image whereas the iSine 20 felt more drastic and like instruments were placed far apart from each other.


Both the Audeze iSine 20 and the Sennheiser HD660S are wonderful products, and although they are obviously very different from each other, they also have a lot of similarities. In general, my biggest takeaway is that the Sennheiser is more neutral and has a big but natural sounding soundstage. The Audeze iSine 20 has more of a U-shaped sound signature with a beautiful and musical sound. It also has a more drastic left-to-right image. If you’re on the go, the Audeze iSine 20 is a great way to bring an audiophile sound along with you.

The Audeze iSine 20 and the Sennheiser HD660S are available for the best price here:

Audeze iSine 20 at Audio 46

Audeze iSINE20 on Amazon

Sennheiser HD660S at Audio 46

Sennheiser HD 660 S on Amazon

Best Audiophile Earphones Audeze iSine 20 vs Sennheiser HD 660 S