The Sivga Apollo Pro is a compact planar magnetic headphone from Sendy Audio. The Apollo Pro launched in mid-2026 and uses circular cups. The driver is 78 mm in diameter and also uses double-sided magnets. The diaphragm is 0.8 microns thick. The typical price is around 499 USD.
The Sennheiser HD 660S2 is an open back dynamic driver headphone released in 2023. It is the latest addition to a proud lineage of headphones that began with the HD 580, followed by the HD 600 and HD 650. They all feature dynamic drivers with a 38 mm diaphragm in a 300 ohm driver. The current price for the HD 660S2 is typically the same as for the Apollo Pro, around 499 USD.
SENDY APOLLO PRO SPECIFICATIONS
- Circumaural open-back design
- Driver: Planar magnetic
- Driver size: 78 mm diameter
- Sensitivity: 93 dB/mW ± 3 dB
- Impedance: 27 Ohms ± 15%
- Cable: 2 m OFC
- Headphone connectors: dual 3.5 mm mono
- Termination: 4.4 mm plug
- Adapters: 3.5 mm SE – 4.4 mm BA and 6.35 mm – 3.5 mm
- Leather carrying case
- Hemp bag
- Weight 354 g
Current price and availability:
- Amazon: Sendy Apollo Pro
- AliExpress: Sendy Apollo Pro
- Sendy Audio: Apollo Pro
SENNHEISER HD 660S2 SPECIFICATIONS
- Circumaural open-back design
- Dynamic driver with 38 mm diaphragm
- Impedance: 300 Ohm
- Sensitivity: 104 dB/1V
- Frequency response: 8-41,500 Hz
- Distortion factor: <0.04% (1 kHz, 100 dB)
- Weight: 260 g
- Velour ear pads
- Cables: 1.8 m (6.3 mm stereo jack) + 1.8 m cable (4.4 mm balanced) + 6.3 mm to 3.5 mm stereo adapter and carrying pouch
- Made in Ireland
Current price and availability:
- Amazon: HD 660S2
BUILD AND DESIGN
Both are well built and comfortable to wear.The Apollo Pro has a very good headband mechanism with a strap; the HD660S2 has a simpler design with padding.
The Apollo Pro has circular hybrid pads that wedge out so that the outer diameter of the pads is larger than the headphone cup diameter. The HD660S2 has oval velour pads. Both use proprietary attachment mechanisms.
The HD660S2 comes with a detachable black straight cable with a 6.35 mm jack. The Apollo Pro has a more fancy, very nice braided cable with a 4.4 mm plug (adapters included).
LISTENING COMPARISON
Associated equipment:
- Amplifier: Topping A90
- DAC: Ferrum Erco mk2
Eye of the Day by Marta del Grandi
The female vocals are much warmer and feel closer to the listener with the HD660S2. The Apollo Pro has more midrange detail, though. It does not feel recessed, just very neutral. It’s also generally more refined across the frequency range.
Teardrop by Massive Attack
Here they are much more similar. The tonal balance is actually quite close; the bass and treble presence is very similar. Vocals are a bit more distant with the Apollo Pro, a bit warmer and thicker with the HD660S2.
The Hidden Camera by Photek
Also here, they sound similar. The HD660S2 is a tad warmer in the mids, the Apollo Pro a bit more refined, but they’re pretty close.
Spontaneous Compositorius by Thomas Agergard
The HD660S2 has a bit more midbass quantity and presents a warmer saxophone timbre; other than that, they are quite similar in tonality. The Apollo Pro sounds very flat in a good way and is generally impressively neutral, with better detail across the board. Imaging is also a bit better.
Sujet Tabou by Renaudin and Cahours
The Apollo Pro sounds very neutral but still has lots of dynamics and detail. The HD660S2 has a warmer sounding guitar and might sound more musical to some, more colored to others.
Outlaws by Bill Frisell
Not too different – both are quite neutral, the Apollo Pro more than the HD660S2, and both sound great.
Dancing in the Dark by Jørgen Dahl Moe
The male vocals are warmer and smoother with the HD660S2. It is more “natural and musical,” but the Apollo Pro is not too far away and is probably truer to the recording. The vocals here often sound a bit dry and clinical on many headphones; the Apollo Pro is neither very dry nor especially warm. I prefer the more colored HD660S2 here, though.
Limit to Your Love by James Blake
The Apollo Pro delivers vocals rich in detail and with a neutral tonality. The sub bass is not light, not overly powerful, but it’s of great quality. The HD660S2 has slightly warmer vocals, slightly less bass quantity, with lower bass quality.
Jambi by Tool
They’re similar but not the same. The Apollo Pro sounds very evenly balanced, the soundstage is large, imaging is good, detail level high.
The HD660S2 is less refined and not as neutral, with a bit more punch from the added midbass but also a bit less detail.
Young Vivaldi by Modo Antiquo
The HD660S2 has beautiful timbre on the strings, very natural and engaging. The Apollo Pro sounds less upfront but with better detail and texture, without sounding dry.
Summer 3 Vivaldi Recomposed by Max Richter
The Apollo Pro delivers a balanced, refined, and nuanced presentation. The HD660S2 is relatively close, but with a bit more upfront mids and a bit less detail.
WRAPPING IT UP
Sound Signature
The Apollo Pro is a very neutral sounding headphone with a slight bass shelf. The HD660S2 has less subbass, more midbass, and a warmer midrange. The treble quantity is similar. However, broadly speaking they’re not too different.
Treble
The treble is very similar in quantity. When it comes to quality, I find the Apollo Pro to be more refined and to deliver more detail.
Midrange
The midrange is more upfront and has a warmer character with the HD660S2. The Apollo Pro is more neutral and delivers more detail than the HD660S2.
Bass
The Apollo Pro has a flatter bass curve. It has relatively more sub bass; the HD660S2 has relatively more midbass. The Apollo Pro has better bass quality.
Soundstage and Imaging
The soundstage is larger with the Apollo Pro; the imaging is better. The HD660S2 isn’t bad, but it’s more intimate and less precise.
Detail, Dynamics, and Timbre
The Apollo Pro offers better detail across the frequency range than the HD660S2.
Both have good dynamics. Depending on the frequencies dominant in the track, one can feel more dynamic than the other. The warmer midbass and midrange can often make the Sennheiser feel a bit more dynamic, but not by much.
When it comes to timbre, I feel the Apollo Pro is the more neutral throughout. The HD660S2 does offer that HD6**-type of midrange smoothness, that slight coloration that makes midrange vocals and instruments often sound more natural. The bass and treble sound more natural with the Apollo Pro.
CONCLUSION
The HD660S2 is a fairly neutral headphone that builds on the legacy of the HD600/650. It does, however, roll off a bit in the subbass and have some very charming coloration in the mids.
The Apollo Pro does not roll off in the bass and has a more neutral midrange. The treble is pretty similar in quantity, which is a good thing.
I like both a lot, but I find the Apollo Pro is more neutral, more detailed, and more refined sounding than the Sennheiser HD660S2. It’s the most technically able and less colored option. The HD660S2 does, however, have a very nice midrange and that HD6** character, which has a lot of fans.
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- Amazon: Sendy Apollo Pro
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- Sendy Audio: Apollo Pro
- Amazon: HD 660S2
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