How much would an additional motor help

With "hum", my general experience is that most times what you think cured it turns out not to have cured it. It finds its way back somehow. Balanced circuitry does away with the problem, completely and forever. Anyway, I hope that is not the case for you; I hope the hum is indeed gone. Also, what some of us call hum is not really hum, in strict terms. Hum is 60Hz or 120Hz pure tone. Is that what you heard? Most of the time, it's noise, a mixture of frequencies, e.g., a buzz.