nSub external earth

I have two front nSats, an nCent and an nSub connected to an nVi. I was getting some low level hum from the nSub and switched it to external earth although I do not have an earth wire from the nsub to the nVi. The nVi is earthed through the power cord. This fixed the hum, but am I running any risk - safety or operational?

MG