I don't think you understand.
They put the I2S tuning on the U18 and they didn't put it on the X18.
Today practically all DACs already come with this adjustment in the setup.
Please don't be angry with me, but please have a look at what i2s is all about.
i2S is a more than 30 year old Philips interface exclusively for direct communication between ICs (on a circuit board) for the audio sector (i2S = Inter-IC Sound). This interface is built into most devices (DACs, CD players, and more) anyway, even if it is not brought out.
Here, however, i2S with LVDS (worldwide industrial standard for the transmission of Inter-IC communication over short cable routes) is used, which is converted accordingly in the transmitter and receiver.
That an HDMI cable is used for transmission is more of a coincidence, but above all it was available and cheap (plugs, sockets, cables). The transmitted signal has nothing to do with HDMI.
What do you mean by i2S tuning?
I had written that Gustard installed a galvanic separation for the USB area in the U18.
I also mentioned the improved and more accurate clock range. That's it!
And you can connect an even better clock generator (C18 $ 1599, -, but also others) to the U18.
The X18 (also applies to the A18) is already very well equipped.
The XMOS XU216 chip does a very good job in the U18 / X18 / A18 when converting USB to i2S.
And that is exactly the point, at this point, after the XU216, all i2S data is already available. The clock can be reprocessed, jitter / noise reduced or removed, but the data for the right and left channel (SD) are ready, as is the separation (WS) for it. The U18 can generate the master clock and make it available to the X18.
It remains to be seen whether this will make an audible difference.
I had 4 x devices with each XU208 and XU216 XMOS chip to try out (Gustard A18, Topping D70 / D70s, D90 / D90 MQA and a prototype).
There was an audible difference between the two XMOS versions for all devices. The XU216 chip brought a bit more resolution, better timing (clearly with "Passion and Pride") and simply more blackness in the background. But we're talking more about per mille than about percent.
Since the different devices sounded different (all very small distances), but the differences between the XU208 / 216 were all very similar, I assume that the XU216 is the real improvement.
I could imagine that the U18 can bring an advantage with DACs with other USB input chips or older XMOS chips.
Since the X18 uses the same XMOS XU216 chip (as does the U18), I am currently assuming similar improvements.
That's why I would first listen to whether the X18 has a sound improvement with the U18.
With the A18 with XU208 the improvement was audible with the U16, with the A18 with XU216 I didn't hear any improvement and even preferred the A18 solo via USB.