^ The copper plate in ZX2 is used for grounding the circuit, it does have an effect on the sound. AK did a similar thing with their AK240SS, where the only difference from regular AK240 is the grounding metal, which changes the sound. At least that's what peoples impressions say.
I would thought grounding the whole circuit and grounding the audio channel are two different things - it will be like saying that using an aluminum chassis on a desktop amp (which also are used for grounding on most desktop amp) gives a 'more balanced' sound than using a steel chassis. Again, I like to have some measurable proof, rather then hearsay or impression, because we all know human listening isn't exactly precise - and if someone is making the claim, (s)he should at least provide some evidence. Plus, I don't trust A&K that much as when they released the AK100, they claimed it is the cleanest sounding DAP in the market at the time - which is true, as someone had measured it on a Audio Precision against other DAP and told me the result, The only problem is that A&K forgot to mention their didn't implement an actual amp section on AK100, but only just put in a couple of transistors and resistor on the line-out to shield it for a direct load and call it the headphone-out, so of course it will be very clean, but also be very much useless and rather deceiving, though not technically wrong by saying it is the cleanest.
A funny thing will be that - by using steel, AK240SS should be a less balanced than the aluminum (*duralumin to be precise) AK240, according to Sony. So which of them is right, I wonder?