HD600, Mojo 1, and the tube-buffer upgrade path

The HD600 baseline

The HD600 is a 300-ohm, open-back, dynamic headphone. Sennheiser lists the current HD600 with a 12 Hz-40.5 kHz frequency response, 97 dB SPL at 1 V, and 300 ohm impedance. Its basic technical identity has remained consistent: high impedance, moderate sensitivity, open construction, and a midrange-first presentation.¹

The consequence is simple. The HD600 does not need huge current the way many low-impedance planars do, but it benefits from voltage swing and a clean amplifier stage. It can play from portable gear, but it scales audibly with better amplification. Its weakness is not that it sounds broken. Its weakness is that it can sound slightly lean, spatially intimate, and dry depending on the chain.

This is why the upgrade question is tricky. A headphone replacement can easily damage the HD600's central strength: believable mids. A chain upgrade can preserve that strength while changing density, perceived bass weight, depth, or smoothness.

HD600 to HD660S2

The HD660S2 is often presented as the more modern member of the HD6 family. It returns to a 300-ohm impedance, unlike the earlier HD660S, and reviews commonly describe it as having stronger low-end extension than previous HD6 variants. Sennheiser's positioning and third-party reviews emphasize bass extension, lower resonance, and a warmer, more saturated presentation.²³

That does not make it an automatic replacement for the HD600. The HD660S2 changes the balance. It adds more weight and fullness, but it also moves away from the HD600's particular midrange neutrality. For listeners who primarily value vocals, acoustic instruments, and tonal correctness, the HD600 may still feel more natural. For listeners who want more warmth and bass without EQ, the HD660S2 may be more enjoyable.

For this reason, the HD660S2 is best understood as a variant rather than a strict successor. It may be worth buying if the target is a warmer HD6-family headphone. It is less compelling if the HD600 is already liked and the desired change is more human, dimensional, or harmonically rich sound.

Mojo 1 changes the question

The Chord Mojo 1 is already a serious portable DAC/headphone amplifier. Chord lists the original Mojo with dual 3.5 mm headphone outputs, 35 mW into 600 ohm, 720 mW into 8 ohm, 125 dB dynamic range, and a very low 75 milliohm output impedance.⁴

Mojo also has a line-level mode. The manual states that pressing both volume buttons while switching the unit on sets the output to 3 V line level for connection to a preamplifier; the setting is not remembered after power-off for safety reasons.⁵

This matters because the Mojo is not a weak baseline. Replacing it with another small DAC/amp does not automatically improve the system. The Mojo tends to sound clean, controlled, and rhythmically precise. With the HD600, that can be technically strong but slightly restrained. The missing quality is usually not more DAC quality. It is often harmonic density, voltage confidence, or a more dimensional amplifier presentation.

Cayin RU9 as a second flavor

The Cayin RU9 is a portable USB/Bluetooth DAC/amp with a KORG Nutube 6P1 tube stage. Retail descriptions and reviews identify it as a compact DAC/amp using dual AKM AK4493 DAC chips and KORG Nutube 6P1 vacuum tubes, with single-ended and balanced output support.⁶⁷

As a result, the RU9 makes sense beside a Mojo 1 only if the goal is a different flavor. It is not best framed as a Mojo replacement. It is better framed as a second portable presentation: warmer, softer, more saturated, and more tube-influenced. For an HD600 owner, that can be musically satisfying because the HD600's midrange responds well to added harmonic density.

The limitation is architectural. Buying an RU9 adds another DAC/amp. It does not solve the cleaner question: how to keep the Mojo as a DAC source and add a tube-like stage without replacing the whole chain.

What a buffer does

A buffer is not a headphone amplifier in the usual sense. Its job is usually impedance matching, signal conditioning, and sometimes harmonic shaping. A tube buffer or Nutube buffer can add texture and tonal density, but it is not meant to be the final power stage for a 300-ohm headphone unless the product is also designed as a headphone amplifier.

The correct chain is therefore:

This avoids the main mistake: expecting a buffer alone to drive the HD600. The buffer provides flavor and interface behavior; the headphone amplifier provides voltage swing, current capability, and control.

iFi micro iTube2 as a buffer

The most relevant commercial example is the iFi micro iTube2. Its manual describes it as usable either as a straight pass-through tube buffer with 1 Mohm input impedance or as a single-source preamplifier with volume control and 100 kohm input impedance. Published specifications list a NOS GE 5670 tube, low output impedance, high signal-to-noise ratio in buffer mode, and wide frequency response.⁸⁹

In an HD600/Mojo system, the iTube2 is attractive because it does not ask the Mojo to stop being useful. The Mojo remains the DAC and possibly the level-controlled source. The iTube2 adds the tube-buffer stage. A clean headphone amplifier after the buffer does the actual headphone driving.

This is more scalable than replacing the Mojo with a new dongle. It also makes the sonic goal more explicit: preserve resolution and timing, add body and harmonic character, then use a proper amplifier to control the HD600.

Nutube and small tubes

KORG's Nutube 6P1 is a low-power dual triode designed for audio use. It was developed by KORG and Noritake Itron, and it uses technology related to vacuum fluorescent displays. AudioXpress describes it as a miniature, low-power, directly heated dual triode. DIY Audio Store lists very low filament power consumption, making it suitable for battery-powered audio circuits.¹⁰¹¹

The advantages are obvious for portable audio: low power, small size, low heat, and tube-like behavior in devices that could not use conventional tubes. The disadvantages are implementation-dependent. Nutube circuits can be microphonic or noisy if poorly designed. A good Nutube device is not just a module in a box; it needs mechanical damping, proper gain structure, sensible impedance behavior, and a clean power supply.

Cheap tube buffers

A buffer sits directly in the signal path. A bad buffer does not merely add harmless flavor; it can raise noise, roll off frequency extremes, create impedance mismatches, exaggerate distortion, or reduce channel balance. With the HD600, the damage may be obvious because the headphone is resolving enough in the midrange to expose grain and noise.

The minimum requirements for a serious buffer are not mystical:

• high input impedance, so the Mojo is not loaded down;
• low enough output impedance for the next amplifier stage;
• quiet power supply;
• clear gain structure;
• real specifications, not only claims about tube warmth;
• good mechanical design if using Nutube, because microphonics matter.

Very cheap inline tube-buffer boards are therefore not a safe recommendation. They may be fun experiments, but they should not be treated as a clean upgrade path for a Mojo/HD600 chain.

A sensible chain

The best version of this system is simple in concept even if it uses several boxes:

The headphone amplifier after the buffer does not need to be colored. In fact, it is usually better if it is clean, quiet, and powerful enough for 300-ohm headphones. The buffer supplies the harmonic shaping. The amplifier supplies control. The HD600 supplies the midrange.

For a portable-only setup, the RU9 remains the simpler choice. It gives a Nutube-flavored DAC/amp in one box. For a desktop setup, a real buffer plus a proper amplifier is the more deliberate and flexible solution.

Where I would land

The HD660S2 is a valid headphone, but it is not the obvious next move for an HD600 owner who already likes the HD600. The RU9 is interesting, but it is a second DAC/amp flavor rather than a clean continuation of the Mojo chain. A real buffer is the most precise answer if the target is to keep the Mojo and HD600 while adding warmth, density, and depth.

The best answer is therefore not “buy a new headphone.” It is:

Keep the HD600. Keep the Mojo. Add a proper tube or Nutube buffer only if there is a real headphone amplifier after it.

That path respects what the HD600 already gets right. It also avoids the common mistake of spending money to change the entire system when the desired change is narrower: not more resolution, but more body.