@tomislavkufrin,
Congratulations!! My SRX-Plus remains one of my favorite builds, even with my Carbon and Blue Hawaii. There is beauty in simplicity, and the SRX-Plus is all of that. As for the hum, I too have had issues with hum in the past, but was always able to get rid of it by 1) using different tubes and 2) elevating the filament voltages, and 3) fine tuning the trim pot settings.
As for elevating the filaments, see my old post earlier in this thread:
Definitely do not give up and accept the hum. I have 2 SRX+ builds, and both had hum at first but after doing some tube swapping and endless fine tuning of the trim pots, both are hum free.
Enjoy!!