Audio/Video, Enterprise, Entertainment, Reviews

Koss Headphones Pro4S: Monitoring With Class


Koss Headphones had a 2014 full of flagship products launched this year, with its sights aimed at an optimistic future. The NFC enabled BT540i simplified Bluetooth listening on mobile devices, then their ergonomic SP330 & SP540 delivered comfortable music enjoyment for all. Finally, Koss will end the year on a high note with its new Pro4S stereophones, geared toward studio and music monitoring.

But what are “studio” headphones, and what makes them different from other models? Most headphones may have a compensated or enhanced EQ to tailor towards certain listener preferences — i.e. more bass. Reference or studio monitoring needs to be free of any tuning, for those who need to listen to an accurate representation of their content, like audio engineers and music producers.

So, we put the Pro4S to the test all month — at home, at a Hollywood recording studio, and on a TV production. Here’s what we found:





The design


The Pro4S is a full-sized, circumaural studio headphone that is a fine blend of transparent sound, comfortable durability, and practical design. Weighing a hair under 7 oz (195 g), even as an over-ear model, it’s compact and light enough for use in any environment.


Its tough, aluminum internal frame is encased by both metal and plastic molding, and outfitted with soft, breathable materials for the best strength and comfort. The headband is supported with memory foam and mesh cloth that feels weightless, even after many hours of wear. The ear cup hinges are also metallic and soft-locking, for added durability without creating temporal pressure.


Comfort and weight are vital for studio applications, where engineers can log many more hours of use than home listeners. That’s why the Pro4S also features the same “D-profile” ear cups first featured on Koss’ classic Pro4AAA, and its recent SP series headphones. Because of this shape, you’ll get great passive isolation that’s evenly distributed more naturally around your ear.  These cast aluminum ear cups have a high quality feel, and are fashioned with a shaved silver trim that may scratch easily, which isn’t a deal breaker for us, but still worth mentioning.


The cups lay flat for storage inside an included soft clamshell case, and the hinges can fold the ear cups towards the headband for further portability. This especially helps for location audio work, where space is limited and mobility is key. The headband is adjustable at discreet steps, and even at full extension, there doesn’t seem to be any signs of stress when bending or tweaking the phones.


Another key feature on the Pro4S is its dual-entry detachable cord. The circuitry automatically routes a stereo signal to both drivers, regardless of which side you plug into. Plus, the remaining open end becomes a pass-through output, so you could daisy chain another Pro4S alongside it.


The included 4.5 ft. (1.4 m) cable is coiled in the middle, in between two long straight sections. This distributes the cable’s weight more evenly, and helps clear up obstructions when plugging into any source. This is also half the length and weight of Sony’s MDR-7506 coiled cable, which is almost 10 ft. long.

While a longer cable could be more accommodating, Koss hit the right mark in length versus obstructiveness, with the added convenience of simply unplugging oneself from the cord entirely. The only downside is if this cable is misplaced, users will have a hard time finding a replacement other than what’s provided by Koss, because of the recessed ports on the Pro4S.

Made with rugged materials, comfortable cushioning, and lined with a simple silver trim, the Koss Pro4S is dressed to impress and made with comfort in mind. Aesthetics and features are great, but does it have the sound signature to match?

The sound


Studio or reference headphones are usually advertised to have a flat frequency response, where, ideally, all signals are reproduced uniformly across all frequencies without bias (example above). The Pro4S is driven by Koss’ new SLX40 elements that are tuned for reference, delivering what they advertise as “incredibly accurate and unbiased sound”.

Its specs seem to be on-par with other studio models, with a few differences:

  • Response: 10 Hz – 25 kHz
  • Sensitivity: 99 dB SPL
  • Impedance: 35 Ohms
  • THD: <0.3% average (@400 Hz – 1 kHz, 95 dB SPL)
  • Max Load: 500 mW

While response and sensitivity are where they need to be, its has a lower impedance and load rating than most models. This is probably to accommodate a wide range of home studio setups and listening, but may have less gain headroom for some professional board outputs.


We lent these stereophones to several engineers in the music and film audio worlds, and their feedback was consistent. The Pro4S, in general, has the soundscape of a studio headphone, but seems to have a character of its own, with additional warmth in the mid range, and clarity in higher registers. However, no frequency bands seem exaggerated, and it retrieves enough detail in all ranges to gauge how tracks sit in a mix.

There are moments when this perceived slight emphasis can sit on the harsh side, when it comes to certain instrumentation. One of the music engineers who tested our Pro4S had difficulty assessing if the twang of an overdriven Telecaster was distorting. Upon switching back to his usual pair, this wasn’t the case.

Despite its accented soundscape, the Pro4S at least provides listeners with a pleasurable experience. For what it’s worth, casually listening to finished music with this pair is actually not as thin or lacking as other studio models. It may not perform like a true “reference” model, but it can at least produce a more neutral sound to a variety of subjective preferences.

The industry


This isn’t to discount the clarity of the Pro4S, which is especially beneficial for tracking vocals. We tested out this pair at a Hollywood dubbing stage, and could properly judge tonal differences between the originally recorded dialogue and the replacement voice-overs. These sessions can last for many hours a day, but there were no complaints of discomfort from the recording team. They especially enjoyed the detachable cord, so they could quickly leave the board for a coffee, without having to take off the headphones entirely.


We also lent the Pro4S to a couple production sound mixers, who primarily record on-set dialogue. Even on an upcoming reality TV series, where there are many mic sources and inputs involved, and characters unpredictably speaking to one other, the Pro4S could retrieve enough detail from all sources to properly EQ and mix. While on location, these headphones are great for long listening sessions, but its materials may heat up considerably on a warm day outside.


Let’s take a moment to compare the Pro4S to two current favorites in the biz — Sony’s workhorse MDR-7506, considered an industry standard, and Sennheiser’s more compact HD25-1, another widely compatible reference pair. All three of these headphones are geared towards professional markets, and the Pro4S seems to be the middle ground that addresses issues with current models.

The 7506’s are considered the standard and used in a variety of applications. It’s also circumaural, with minimal pressure around the ear, and a neutral soundscape that is uniform from production to post. The earcups fold inward towards the headband, but because it has a much longer cable, the 7506 can easily become a tangled mess during storage.

The on-ear HD25’s are more compact and also great for many situations, especially while on location. They have great isolation, and seem to distribute pressure evenly across the adjustable headband. However, the on-ear style can become fatiguing during long sessions, especially if the listener is wearing glasses.

Take the isolation and compactness of the HD25’s, with the sound and comfort of the 7506, add a detachable cable, and the Pro4S is a nice marriage between the two.

The verdict


Not only is the Pro4S quite possibly the most comfortable pair of headphones of its class, it has class too. It may have a sound signature that slightly deviates from a promise of “incredibly accurate and unbiased sound”, but as always, Koss’ functional design and build quality places the listening experience as a whole first and foremost. At $149 USD, the Pro4S is worth experiencing, no matter where your studio happens to be.