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We compare products from their published specs and features; the verdicts below are our independent editorial opinions.
Updated: 2026-09-25 · 6 min read · Independent comparison
• HE5XX biggest strength: Open-back, dual-sided planar design with 2-micron Nano Diaphragm membranes and a stated 15 Hz–50 kHz range — a bright, neutral tuning aimed at detail and air.
• HE5XX biggest trade-off: Open-back cups leak sound both ways, so it is a poor fit for noisy rooms, shared spaces or blocking outside noise.
• T50RPmk4 biggest strength: Fourth-generation RP planar driver with very fast response and a symmetrical housing design that Fostex says improves left/right matching for cleaner imaging.
• T50RPmk4 biggest trade-off: The data sheet lists frequency range only as "wide range" and gives no impedance figure, so buyers get less hard spec to compare against the HE5XX.
• Pick HE5XX if you want an ultralight open-back for detail-first listening at a desk.
• Pick T50RPmk4 if you want a studio-lineage closed planar tuned for gaming localization and marathon sessions.
The HE5XX is a planar magnetic over-ear with an open-back enclosure, and that single design choice shapes everything about it. Open backs let air move freely behind the driver, which is the classic route to the kind of flat, uncolored frequency response people mean when they talk about neutrality — less cup resonance, less boxy coloration, more sense of space. The 2-micron-thick Nano Diaphragm membranes and double-sided magnet array are aimed squarely at that goal, and the stated 15 Hz–50 kHz range tells you the driver is expected to be quick and extended rather than warm and rolled off.
The other headline is weight. At 354 g with an aluminum, faux-leather and plastic build, this is a headphone that has been deliberately slimmed down — Drop states it is 30 percent lighter than the Legacy HE500 it descends from. That matters more than it sounds for neutrality: a light headphone stays comfortable through long listens, and comfort is what actually lets you hear a flat response properly instead of fighting the fit. The 18-ohm impedance also makes it comparatively easy to drive, and the 3.5 mm jack with an included 6.35 mm adapter covers both portable and desktop gear. There is also balanced-cable wiring for anyone who wants to push it further.
Who should buy it: listeners who want an open, airy, neutral-leaning planar for desk work, mixing references or long critical listening sessions, and who value low weight and easy amplification. Who should skip it: anyone who needs isolation — on a train, in an open office, or next to a noisy PC — because the open-back design leaks in both directions, and the spec sheet lists no noise control at all.
The T50RPmk4 comes from a very different place. Fostex has been building RP-technology planar headphones for 50 years, and this fourth-generation driver is sold on one idea above all: overwhelming response speed. Speed is exactly what a flat frequency response needs to be useful — a driver that starts and stops fast preserves transients and separation instead of smearing them, which is why Fostex frames this as delivering "excellent localization and separation." For gaming, that is the whole ballgame: you want to place a footstep, a reload or a voice precisely, not just hear it.
The design detail worth flagging is the symmetrical left and right housings. Fostex says this reduces the auditory difference normally caused by whether a connector or internal part sits on one side, letting the two channels match more closely. In plain terms, a more even left/right response makes the center image more solid and positional cues more trustworthy — again, a localization feature. The new thicker, low-resilience around-ear pads are the comfort half of the story, and Fostex explicitly ties them to long gaming sessions.
Who should buy it: gamers and studio-style listeners who want fast, well-separated planar sound with tight channel matching and pads built for multi-hour wear. Who should skip it: spec-sheet readers who want published impedance and a numeric frequency range to compare against — Fostex lists the range only as "wide range" and omits impedance entirely, so you are buying on the design story and driver lineage rather than hard numbers.
• Best for a neutral, open, airy presentation → DROP + HIFIMAN HE5XX (open-back enclosure, 15 Hz–50 kHz).
• Best for gaming localization and separation → Fostex T50RPmk4 (fast RP driver, symmetrical housings).
• Best for long-session comfort → Fostex T50RPmk4's thicker low-resilience pads, though the HE5XX counters with a genuinely low 354 g weight.
• Best for easy amplification → DROP + HIFIMAN HE5XX, thanks to its published 18-ohm impedance.
• Best for spec transparency → DROP + HIFIMAN HE5XX, which lists impedance and a numeric range; the T50RPmk4 does not.
• Best for quiet rooms and shared spaces → Fostex T50RPmk4, since the HE5XX's open back leaks sound both ways.
• Best for balanced-cable upgrades → DROP + HIFIMAN HE5XX, which is wired to accept a balanced cable.
• Best for buyers who want more published data to reason from → DROP + HIFIMAN HE5XX.
If I had to buy one, I would take the DROP + HIFIMAN HE5XX. In my opinion it is the more useful flat-response tool for most people: an open-back planar with a stated 15 Hz–50 kHz range, an easy-to-drive 18-ohm load and a 354 g build that Drop says is 30 percent lighter than its ancestor. That combination of neutrality, low weight and amplification flexibility is hard to argue with for desk listening. I would pick the Fostex T50RPmk4 instead if gaming localization and marathon session comfort are the priority, because its fast RP driver and symmetrical housings are built for exactly that — but for a broad, honest flat-response daily driver, the HE5XX wins my vote.
does frequency response matter in headphones
Yes — it describes which frequencies a headphone emphasizes or downplays, which is the core of what "flat" means. A flatter response means less coloration, so you hear the recording rather than the headphone. The HE5XX publishes 15 Hz–50 kHz; the T50RPmk4 lists only "wide range."
Open-back designs like the HE5XX let air move behind the driver, which supports a more neutral, spacious sound but leaks audio in and out. Closed designs isolate better and are the usual choice for gaming and studio work. The T50RPmk4's symmetrical closed housings are aimed at precise channel matching.
Not always, but it helps. The HE5XX's 18-ohm impedance makes it comparatively easy to drive, and it is wired for a balanced cable if you want more headroom. Fostex does not publish an impedance figure for the T50RPmk4, so there is no comparable number to work from.
On the specs, the T50RPmk4 is the gaming-oriented pick: Fostex markets its fast response speed for localization and separation, plus symmetrical housings for tighter channel matching. The HE5XX's open-back design is better suited to detail listening in a quiet room.
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