The Tubulus is not an HDMI cable, but an I2S cable using HDMI connectors (similarly as there are I2S cables using RJ45 connectors).
So, what‘s the difference? In order to transfer I2S over a cable, you need 12 lines, which are soldered to pins 1 to 12 of the HDMI connector (so far at least vendors agree upon). This leaves pins 13 to 19 of the HDMI connector for other purposes. Some vendors use these for additional „convenience“. This is where all agreement ends. PS audio seems to use one of them to communicate the preemphasis bit (pin 15?) from the DMP to their DAC. Other vendors may use them differently.
This leads to all sorts of dilemma. As an owner of matching devices of the same vendor, you might want to use a real HDMI cable, i.e. one that connects all pins of the HDMI connector, such that you get the full functionality out of your gear. However, such a cable might not be optimized for I2S and audio. In order to switch to an I2S cable, you actually need to find out which pins your source and DAC combination relies on, and then check with the vendor of the cable, or order it according to your specification (which Tubulus offers). Alternatively, you can rely on trial and error.
If you combine devices of different vendors, it might actually be of advantage to have a real I2S cable, omitting the connection of all or some of the non-I2S pins. This way you incompatibilities between different vendors. For such an example you might want to look at
Holo Red impressions. You might see what kind of dilemma cable manufacturers are in.
Regarding the Mk2, the sound quality should not be affected, unless you want to play some of the very few CDs that featured the infamous preemphasis. You might however miss some convenience, like green light instead of a red.