Alexey,

Yikes.  It really must be first understood that RME strives to provide it's users with the most Linear, and Low Noise/Distortion interfaces that Technology will allow.  Tubes?  Enticing, and Heart Warming as they seem...Notsomuch.  Never were.

For such requests, it might be easier to install some Tube Distortion/Overdrive/Compression Plug-Ins to your Work Station.

In the Home Listening environment, you might consider bloating the Bass a little, bumping up the Mid-Range, and rolling off the Trebles a bit with your PEQ.  In terms of Frequency Response, this may more closely replicate the non-linear behaviors of Tubed Gear.   Throw in a few clicks of Channel Imbalance to enhance the effect.

I, and no doubt a majority of other users cannot/will not "endorse" such butchery to our music playback, but if non-linear is what you truly seek...

*So that I'm not the BAD GUY in this exchange, allow me to say that I do understand what you wish to hear.  To that end. I realize now that what we were doing years ago was simply masking one layer of distortions in our playback, with yet ANOTHER, even LARGER
layer of distortions that we found more pleasurable...yet never so much as 1mm closer to the truth.  The often VERY COSTLY act of Self Deception in so many words.

Nonetheless, do as you like.

Have fun!

Curt