i just upgraded my audio mixing capabilities of my system. i have 2 TASCAM VL-X5 speakers connected to EURORACK UB502 (to control the volume) and from that mixer through MOBILE Pre preamp/audio interface (to control the gain of the XLR input for mic and control the volume of the headphones) to computer (through USB). Everything works fine except when i run the system for too long i start getting this weird hiss from the speakers. Is it normal or is there something wrong with my mixer?

Also i noticed that if i disconect one of the speakers the hiss is gone.