We compare products from their published specs and features; the verdicts below are our independent editorial opinions.
Updated: 2026-09-18 · 8 min read · Independent comparison
If you want a closed-back, over-ear studio monitor that isolates you from a noisy room, choose Superlux HD661; if you need an in-ear planar IEM with a genuinely headphone-like sense of scale, choose Linsoul Kiwi Ears Aether. That's the whole decision in one line, and the rest of this page is the reasoning behind it.
• Superlux HD661 — biggest strength: a sealed, closed-back over-ear design built for monitoring in loud environments, with a 40mm dynamic driver, 68Ω impedance and 102dB sensitivity.
• Superlux HD661 — biggest trade-off: it's a full-size plastic over-ear, so it's bulky, it's wired only, and it does nothing to hide the gear you're plugging it into.
• Linsoul Kiwi Ears Aether — biggest strength: a 15.3mm planar diaphragm driven by multi-layered N52 neodymium magnets, which is a very large driver for an in-ear and is designed to sound more like a full-size headphone than a typical earbud.
• Linsoul Kiwi Ears Aether — biggest trade-off: it's an in-ear, so isolation depends entirely on how it seals in your ears, and the package leans on the included Kiwi Ears Terras cable rather than on any user-replaceable tuning.
• Buy the Superlux HD661 if you need an over-ear you can wear for long studio or broadcast sessions and use in a noisy room.
• Buy the Linsoul Kiwi Ears Aether if you want big-driver planar sound in something that disappears into your pocket.
The HD661 is a sealed studio monitor. That word "sealed" is doing most of the work here. Superlux describes it as an enclosed design with an airtight build, explicitly intended so you can work comfortably in areas with high external noise. If you've ever tried to track vocals or monitor a mix while a drum kit, an air conditioner or a busy room is competing with your headphones, you know why that matters. The closed back isn't a lifestyle choice — it's the point of the product.
Inside, it's a 40mm dynamic driver with a neodymium magnet, rated 10Hz to 20,000Hz, with 68Ω impedance and 102dB SPL sensitivity at 1mW/1kHz. In plain terms: 68Ω is on the higher side of easy-to-drive but not punishing, and 102dB sensitivity means it doesn't need a monster amplifier to reach a usable level. A laptop, an interface headphone output or a mixer will all drive it. The 10Hz bottom figure tells you Superlux is claiming real low-end extension, and the stated design goal is a balanced, high-resolution tone for engineers working in recording and monitoring.
The accessories tell you the intended use as clearly as the specs do. You get a 3.3 ft (1m) cable, a 9.8 ft (3m) cable, a gold-plated 3.5mm-to-6.3mm adapter, a cable clip and a carrying pouch. That's a monitoring kit: the short cable for a phone or portable player, the long one for reaching across a desk or a live stage, and the adapter so the same headphone works on pro gear with a quarter-inch jack. The clip keeps the long cable from snagging on a chair or a mic stand. Nothing here is decorative.
Who should buy it? Someone who needs one closed over-ear that works at a desk, in a booth and on a stage, and who values isolation over portability. Who should skip it? Anyone who wants to commute with something that vanishes into a pocket, or who wants wireless freedom — the HD661 is a wired, plastic, over-ear design, and at 6.72 ounces it's a real object sitting on your head. Note the listing also carries a Bluetooth field, but the connectivity technology is wired with a 3.5mm jack; treat this as a wired headphone and don't expect wireless convenience from it.
The Aether is the opposite philosophy in a much smaller package. Kiwi Ears built it around a 15.3mm planar diaphragm — which the brand describes as the largest IEM planar driver it has made — paired with multi-layered N52 neodymium magnets. Planar drivers move a flat diaphragm rather than a cone, and the reason that matters at this size is control: the Aether is explicitly designed to output a "truly headphone-like sound" rather than the thinner, more compressed presentation people often associate with cheap earbuds.
That phrase is the whole pitch, and it's a useful one. If you've used small in-ears before and found them shallow — no weight in the low end, no sense of space around instruments — a large planar driver in an IEM shell is the answer to exactly that complaint. The Aether's job is to give you the scale of an over-ear in something that fits in your ears.
The package also includes the Kiwi Ears Terras cable, described as 4N single crystal copper, 32AWG, four cores. Kiwi Ears frames the cable around clean, noise-free transmission and clarity. I'd treat the cable as a build-quality and ergonomics upgrade more than a magic wand, but it is a genuine part of what you're buying: a single-crystal copper four-core cable is a nicer thing to live with than the thin stock wire that comes with most budget IEMs, and it signals that this is aimed at people who already care about their chain.
Who should buy it? Someone who wants one wired IEM for music on the go, at a desk, or as a compact alternative to a full-size headphone, and who specifically wants a big planar driver rather than a conventional dynamic one. Who should skip it? Anyone who dislikes things in their ears, anyone who needs the passive isolation of a sealed over-ear in a genuinely loud room, and anyone who wants the wear-and-forget simplicity of a headband design. An IEM lives or dies by its fit, and no spec sheet can tell you whether it will seal in your particular ears.
Side-by-Side: Which Should You Choose?
• Best for long listening and work sessions → Superlux HD661. An over-ear with a 6.72 oz plastic frame and a choice of a 3.3 ft or 9.8 ft cable is easier to wear for hours than anything inserted into your ear canal.
• Best for portability → Linsoul Kiwi Ears Aether. It's an in-ear with a four-core 32AWG cable — it goes in a pocket, and the HD661 never will.
• Best for low-end ambition in a small package → Linsoul Kiwi Ears Aether. A 15.3mm planar diaphragm is unusually large for an IEM and is built to deliver headphone-like weight.
• Best for pro-gear compatibility → Superlux HD661. It ships with a gold-plated 3.5mm-to-6.3mm adapter, so it plugs into interfaces and mixers without a trip to the store.
• Best for cable quality out of the box → Linsoul Kiwi Ears Aether. The included Terras 4N single crystal copper cable is a step above typical bundled wire.
• Best for pure simplicity → Superlux HD661. Wired, closed, one driver, two cables, an adapter and a pouch. Nothing to fiddle with.
• Best fit for an existing IEM habit → Linsoul Kiwi Ears Aether. If you already own IEMs and want a bigger-driver upgrade in the same form factor, this is the natural move.
If I had to buy one, I'd take the Superlux HD661. In my opinion it's the more versatile tool: a sealed over-ear that works at a desk, in a booth and on a stage, comes with the long and short cables and the adapter you need for pro gear, and doesn't depend on getting a perfect seal in your ears to sound the way it's supposed to. The Aether is the more interesting product on paper, and I'd recommend it without hesitation to anyone who wants planar scale in a pocket-sized wired IEM. But if you want one headphone that covers monitoring, isolation and long sessions, the HD661 is the safer purchase. Check the current price on Amazon for both before you commit — the value gap can shift, and that's the one number I won't guess at.
FAQ
For monitoring in a loud room, yes — in my opinion. The HD661 is a closed-back, sealed over-ear designed for exactly that, with two cable lengths and a 6.3mm adapter included. The Aether is an IEM, so its isolation depends on how well it seals in your ears.
There's no impedance or sensitivity figure published for it, so I can't give you a hard answer. Planar drivers generally respond well to a bit more clean power, but nothing in the specs says it's hard to drive. If your source sounds thin, that's when a small amp is worth trying.
The Superlux HD661, most likely. It's an over-ear at 6.72 ounces with a headband, so nothing sits inside your ear canal. The Aether is a large-driver IEM, and comfort there is entirely about tip fit and whether you personally tolerate in-ears for hours.
They're solving different problems, so "better value" depends on your use. If you need isolation and pro-gear compatibility, the HD661's included cables and adapter make it the more complete kit. If you need portability and a big planar driver, the Aether's Terras cable is a strong inclusion. Check the current price on Amazon for both and decide from there.
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