Hello guys
I have recently bought a pair of Adam A5X speakers, using it with Schiit Asgard 3 Amp, plugged into MOBO (AB350 Gaming 3) integrated sound card ( * ALC 1220 120dB SNR HD Audio with Smart Headphone Amp).
My friend advices me to buy an additional sound card (something from Asus Xonar) because my speakers do not offer their full potential without subwoofer and a good sound card
What would you advice?
Thank you for your advices 
Don’t get a soundcard (at least not Asus, Soundblaster, etc.).
A proper DAC or Audio Interface (depending on what you are looking to do) are better.
Can’t comment on the need for a subwoofer.
I believe the vast majority of people here will recommend an external DAC over a sound card any day. A computer is an EMI generator and getting your audio out of it as quickly as possible is almost always better. One of Schiit’s DACs should pair well with the Asgard 3.
The only thing I can imagine a soundcard being useful is for a optical out, and considering things like a Topping d10s exists even this use case is arguably bad.
As for subwoofer, can’t comment on the need, but I would say to look for solutions like the Emotiva 8s/12s which allows you to cut the lower frequencies before passing to the speakers.