How could I improve my headphones setup, what to replace?

[audiobomber]

I don't know how good or bad the DAC is on the 2i2. Clearly the headphone section is grossly underpowered for your headphones. For example the Schiit Magni 3+ can put 410mW into a 300Ω load. The Scarlett can only output 10mW into the same load. You absolutely need a headphone amp, not sure about a DAC. 

 

I'm a big Schiit Audio fan and would recommend either the Magni 3+ or Magnius for your application. The Magni can power headphones from the front panel headphone jack and speakers via RCA outputs on the rear panel. Plugging headphones in mutes the speaker outputs. 

 

The Magnius has a bit more than double the power of the Magni and somewhat higher sound quality, for double the price. The Magnius has both XLR and RCA outputs. The headphone jack does not mute the rear outputs.  

 

Schiit has a range of DAC's if that is of interest. Like I said, I don't know the quality of the Scarlett DAC but I know the Schiit DAC's and they are awesome for the price. I am currently using a Modi 3 in my headphone system and will be ordering a Modius later today.

 

Schiit also has the Asgard headphone amp, which has a power rating in between the other two (600mW into 300 ohms) with available DAC module for $299. 

 

https://www.schiit.com/products

 

iFi Audio's Zen is another possibility. I own several pieces from both companies but nothing from the new Zen line. I trust the value of Schiit a bit more than iFi, but both make some great products. 

[audiobomber]

25 minutes ago, Zarathustra said:

I still will be needing Focusrite to connect USB and serve as audio-interface for PC and "on top of that" the Focusrite output goes to Schnitt Magnius (for example) ?
 

Yes, the 2i2 will convert the digital USB signal to analog. Connect the analog outputs on the rear of the 2i2 to the RCA inputs of the new headphone amp.

 

With the Magni, connect the Magni RCA output jacks to the RCA inputs on your speakers. If you are using a Magnius, you could connect from the Magnius single-ended (RCA) outputs or from the balanced (XLR) outputs of the headphone amp to your speakers. Set the 2i2 to fixed level if that is available. If not, set it partway and control the level with the headphone amp.

[audiobomber]

I've not heard one, but the iFi Audio Zen Can is well regarded.