"Technically I have pretty good idea what's causing it" - Ehm, I guess my phrasing wasn't very exact. It should've been like: I know this is pretty typical behavior for amplifiers, but I'd like to know if it's normal for the Z-4. ^^;
My guess is that it's because the operating voltage of the amplifier is dropped to zero immediately when powering off, and this, for some reason, causes the woofer to get a "burst of unstable voltage"(what? : D) as the capacitors* in the power source and amplifier are emptied suddenly. It would probably be audible from the main speakers too, but they're uncapable of producing that low sounds loud enough.
In most separate amplifiers this behavior is eliminated by designing the thing in a way, that prevents this behavior. tehee~ >_>'
Aaand the reason why I think it's 50 Hz - well, in addition that it sounds like it -, is because the mains current we get from wall outlet is 50 Hz Alternating Current. Oh and around here we have 230 V voltage in mains, so maybe the power source has a little design flaw compared to the US model?
*In case you don't know what a capacitor is, think it like this: Imagine an empty bucket. Now when we plug power in it(yes, we plug power in the bucket), it fills up and works as a reserve. If the subwoofer's amplifier happens to be under heavy use, it can take large amounts of power suddenly and that power is then drawn from the capacitor(s), or bucket(s) that are part of the power supply inside the Z-4's subwoofer case. In other words, the capacitors in power supply help the current sent to the amplifier to be stable.
There are other uses for capacitors too, but I think and hope, that this explanation was fitting in this case.
***By the way, I'm pretty sure that the sound quality has improved a bit now after using the speakers for a while, or "burning them in". And overall, now after having listened them more, I think the mids are pretty nicely audible especially if the Bass level is turned at ~1/3th or a little more instead at half(or more : o).