Firmware version wouldn't affect a hiss, although it's worth checking to see what version is installed anyway.

Hiss can be the result of a variety of issues. Relentless's example is one: other devices (curling iron, dimmer switch, refrigerator compressor). I'd also check the layout of the wiring - power cords run too close to interconnects or speaker cables can introduce noise. If the sound is volume-dependent, I'd look at interconnects in particular. Relentless's question about which speakers it affects is a good one.