@mintakax The only benefit you will get from plugging it into a 20A circuit is that likely that circuit would be a dedicated line, so it would be isolated from other noisy electronics on the same circuit.
I am an authorized T+A dealer, and I can tell you that the most powerful fuse in the unit is 10A, so it for sure never draws more than 15A. If anything, it actually only uses less than 5A continuously.
Actually for both my own T+A A 3000 HV and PS 3000 HV, I replaced the 20A IEC with 15A ones. This opened up my ability to choose from many more available power cables with 15A connectors.
And in case you ask, no, a 20A cable does not deliver more current than a 15A cable if it is not needed. Quality high current cables (e.g. 10AWG conductors) can do wonders to help that amp sing.
Congrats on your purchase of a fantastic amp.