The Xduoo MT-604 Hybrid Tube Amplifier is a very compact desktop headphone amplifier, it's size is just a little longer
than a regular smartphone and from the thickness about 3 smartphones pilled over of each other.

The device comes in a small very basic packaging. In the box you get the device itself, an external power-brick and cord, a warranty
card and a manuel in Chinese language only. The device is priced at 160€ so it's not a very expensive amplifier. The 4 Type 6J1 tubes
come already preinstalled in the device and are protected with rubber-like caps. The build quality of the unit is very solid
considering the price.

The hybrid design of the MT-604 combines vacuum tubes for the preamp stage with solid-state components on the output stage.
The unit is a balanced only design meaning there no RCA connections available at the back, neither 3.5mm or 6.35mm headphone sockets, so
bare in mind that your headphones either need a 4.4mm pentaconn or 4-pin Xlr cable connector and the DAC needs to have balanced sockets
to run on this device. Power-wise this amplifier can deliver up to 2 Watts on 32 Ohms which is a lot.

Controlling the volume of the device is not an easy task because there are 2 separate volume knobs for each channel, so
unless the DAC connected has volume control it will be a bit challenging to get exact volume matching for both channels, a bit
of an odd design choice from the manufacturer.

A bit disappointing is also the fact that the supplied tubes don’t really produce any glow so instead they simply added LEDs at the base of the vacuum tube
sockets to "fake" the glow, i noticed this because normally tubes will slowly start to glow when warming up and slowly fade out when turning
down the unit which is not the case here.

So after all this criticism what about the sound, well it is a bit of trade of also here. One thing out of the way i wouldn't recommend using
the MT-604 with IEM's as there is quiet noticeable background noise. With full-size headphones no problem but IEM's no, not really.
But once connected to a matching headphone the MT604 is able to deliver a really great experience it is impressive in the low-end and midrange
adding warmth and sweetness, not to the point that it comes close to a "real" tube amp but hinting in that direction. It's
also open sounding, layered and quite spacious, on the other hand there is some lack of detail retrieval and transparency in the upper region.
I was able to get great synergy with high impedance headphones like the Sennheiser HD660s2 and HD620s, it seemed to add bass
impact to 660s2 and opened the soundstage of closed-back HD620s a bit further. To my surprise this amp managed to drive the Hifiman HE6 not to its full potential but
quiet close to it, with the other planar headphones it sounded ok.

I will skip the comparions this time because i simple don't have any other desktop amplifier in this price range to compare it with and i think this
is a niche product

Last Word:
So final thoughts...., it really is a niche product, it's not very user friendly, there are some obvious limitations with equipment pairing
due to design choices that were made. A balanced Dac with volume control is a must have so that both knobs on the MT604 can be maxed out and the DAC will
take over with volume duties. What remains is a warm, lush & lesser detailed relaxed presentation that can end in great synergy with specific
headphones. I think this really is for experienced hardcore users or for those that have limited budget and want to get glimpse of a tube-like experience
without breaking the bank.