In this review, I’m looking at the $169 xDuoo MT-604 balanced tube headphone amplifier. The MT-604 has 4x 6J1 tubes, a true class-A buffer and high output power.

About xDuoo

Shenzhen xDuoo Technology Co., Ltd. is a Chinese developer and manufacturer of affordable high-quality audio products. Their team of experts has made over 100 products to date including DACs (Digital-to-Analog Converters), DAPs (Digital Audio Players), amplifiers and Bluetooth devices.

Disclaimer: This sample was provided by Shenzhen Audio for the purpose of an honest review. All observations and opinions here are my own based on my experience with the product.

xDuoo MT-604

Specifications

• Power supply: DC12V/2A
• Output power: 2000mW (1kHz, 32Ω load)
• Dynamic range: 85dBA (32Ohm)
• Frequency response: 10Hz~60kHz (±0.5dB)
• Suitable headphone impedance: 16Ω ~ 600Ω
• Gain: +15dB
• Size: 17*10*6cm
• Weight: 0.55kg
• S/N: 115dB
• THD+N: ≤0.01% (1kHz, 32Ω load)

Design

The exterior of the xDuoo MT-604 is a dark grey, sand-blasted aluminium alloy chassis. The side and top panels are perforated to improve cooling. On the top of the unit are the 4x 6J1 tubes. I’m not sure why the tubes are sunken into the panel but it’s a bit of a shame as they cast a lovely orange glow when powered.

Around the back panel are the balanced XLR inputs and the 12V power input. On the front panel are the two individual volume pots, the 4.4mm and 4-pin XLR balanced outputs plus a 4.4mm balanced input.

There are four silicone feet on the bottom of the chassis to prevent slipping and protect your desktop. The build quality is superb and the unit looks and feels like a high-quality device. I especially like the anodized red volume pots which contrast really nicely with the dark grey finish on the chassis.

My single gripe with the design is that the notch on the volume pots is very hard to see, making it difficult to adjust the volume evenly. Furthermore, it’s very useful to be able to see the volume setting at a glance so that you don’t accidentally destroy your headphones or your ears.

Internals

Internally, the MT-604 has Rubycon capacitors, which are reputed to have a smooth, warm sound plus a good high-frequency response. The four 6J1 (6AK5) act as a preamp to the transistors used in the class-A buffer output stage.

The MT-604 aims to provide the best of both worlds by utilizing tube and transistor output stages: tubes are known for their warm, smooth and organic character. Transistors, on the other hand, have high speed and large dynamics resulting in improved resolution, as well as more driving power.

Output power is pretty impressive, pushing 2 watts with a 32Ω load. That’s plenty of juice to drive all but the most demanding headphones. None of the headphones currently in my collection was able to even make the MT-604 break a sweat.

One feature that I really love about this amp is the built-in mute circuit. It helps to protect your headphones from those annoying pops that tube amps often exhibit. So don’t be concerned if you don’t hear any noise right away: it takes about 20 seconds for the relay to open up. Not only can the pops be frustrating, but they can also damage your headphones. Another plus is that this gives the tubes some time to warm up so they’ll perform more optimally when you start listening.

Sound

For the source, I used my Gustard A18 DAC with balanced XLR cables. Headphones tested include the Acoustic Research AR-H1, BLON B20, ThiAudio Phantom and Hifiman Sundara. Earphones used for testing were the TRI Starsea and FiR Audio 5×5.

All testing was done using the included stock tubes. There are a bunch of alternative tubes for this amp available but tube rolling is a rabbit hole I haven’t gone down (yet). As such, I’m looking at the xDuoo MT-604 from the perspective of someone who is just getting started with tube amps or someone who wants to use it out of the box in its default configuration.

I was sceptical about this amp due to its basic functionality and low price but overall, I’ve been really impressed with the sound quality. The signal is clean and thanks to the solid-state output stage, there’s no audible distortion and there’s loads of power on tap.

As far as the tube stage goes, it’s surprisingly transparent which combined with the solid-state output results in a detailed, clean sound. But there’s a definite layer of subtle richness and warmth added that works particularly well with faster or leaner headphones. Furthermore, the high frequencies are slightly rounder, taking off some of the harder edges and resulting in sweeter treble notes.

End to end reach is good with deep lows and ample treble extension. MT-604 adds a little body and musicality while maintaining speed, drive and dynamics. This amp is an excellent companion for a leaner or neutral DAC if you want to sweeten up the sound without compromising on technical performance.

Select Pairings

Acoustic Research AR-H1: MT-604 is the perfect partner for my Acoustic Research AR-H1 headphones. The AR-H1 has a very clean, precise sound but it can be a little sharp in the treble. Matched with the MT-604, the highs are smoother and sweeter while the transient speed and resolution are left intact. The bass extension is fantastic and I wish more people had the opportunity to try not just these headphones but this headphone/amp combo.

BLON B20: This is another great pairing for the MT-604. The B20 doesn’t have as much treble energy as the AR-H1 but it has a lively upper midrange. Paired with the xDuoo, the B20’s upper mids fill out ever so slightly making the overall sound a tad smoother. In fact, I spent several hours with this combo after my initial critical listening because the headphones and amp work so well together.

TRI Starsea: I wanted to test the Starsea with the MT-604 because it’s a low-impedance (9.5Ω) and fairly sensitive IEM. I didn’t hear any background noise, even with the amp’s volume turned way up. The sound is clear, dynamic and lively with a touch of extra sweetness. The bass extension is good and the Starsea retains its tonal balance but has a little added musicality.

FiR Audio 5×5: The 5×5 isn’t a particularly sensitive IEM but it has a sparkly, crisp treble that I wanted to hear with the MT-604. The 5×5 is already amazing (in my opinion) but with this pairing, it sounds devilishly good. There’s no background hiss, the bass is full but clean and the treble is a tad softer but still loaded with detail. I could listen to this all day (and probably will).

Conclusion

I’ve been impressed with xDuoo products in the past but I hadn’t heard any of their standalone amplifiers until now. The xDuoo MT-604 is a great little device and would be the perfect choice for anyone who wants to get started in tube amplifiers. It does have some limitations (only balanced inputs and outputs) but that’s how they were able to keep the price down, which is a win for the people.