General Information

I received this product for free in return for an honest review. All thoughts and opinions given are my own.

Testing Details:
I have tested the Fosi Audio Box X4 with my HE400SE, DT770, and DT990.

When playing music from a digital source I used the Topping DX1 as my DAC.

When playing music from a turntable I used a Sanyo TP-1020 direct drive Turntable with a rubber mat.

The Fosi Audio Box X4 used ample packaging to make sure the product arrived without damage. The packaging was not difficult to open or in excess. The tubes were shipped in their own separate smaller boxes within the primary box containing the product.

Assembly and Setup:
Assembly and setup was very simple and easy. The Box X4 uses JAN 5654W vacuum tubes which are quick to install. Following that the Box X4 plugs into a standard wall outlet. Depending on your listening setup you can either connect it to your media source using a 3.5mm aux plugin or RCA plugins.

Build Quality:
I have no issues with the Box X4 regarding build quality. The construction seems to be pretty similar to most other products of its type in this price range. The Primary body/shell is made of metal with plastic plates on the front and back. The dials for adjusting treble, base, and volume are solid and not loose in any form.

Usage and Testing:
My first test was using the DT770s (80 Ohm). The audio quality was good and consistent with no major issues.
Following the DT770s I attempted using the HE400SE. This resulted in a very different experience. The audio was sometimes decent, but never anything spectacular. Very frequently the audios volume would leap up and down, as well as become more tinny and more condensed at seemingly random. This has never been an issue for either my Objective 2 or my LG Quad DAC while using the same headphones with the same cable.
Up next came the time to test the DT990 (250 Ohm). My guess was that the audio issues encountered while using the HE400SE were related to impedance but this seems to be incorrect. The DT990 performed equally to the DT 770, with clean sound and no issues.
Throughout all pairs of headphones the treble and bass dials made a nice quick way to slightly adjust the output without going too heavy on modification.
Unfortunately I am at a loss as to the issues with playback through the HE400SE. I only have one pair of planar headphones so I cannot compare a second pair. All the dynamic driver devices I've tested play back fine however.

Review Pros:
+Many different connection options for diverse use cases.
+Solid construction.
+In most cases clean enough audio with no major issues.
+Aesthetically unobtrusive.
+Powers 250 Ohm headphones.

Review Cons:
-Weird interaction with HE400SE?
-Power cable could be longer.


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