In some threads it is reported that ZP80, ZP90 and connect products sometimes looses phone output level. I have a ZP90 that had the same problem. I have extended the life of the ZP90 as I realised that the optical output on the box is still working. 

So, using the optical output is a solution to both the outpur power problem and the problem of the same components of electrical hum and noise in the phono outpu signal.