It's absolutely true, that this is an issue in very high end monitor speakers as well. I have Genelec 8020B monitors and Genelec is regarded as a cream of the crop, these were back then their smallest model and they were
350€/piece.
They have completely custom power-supply and amplifier design and they're really low noise, but you can still put your ear to it and hear hiss. It's negligible though.
One of my friends has larger versions and they're many times more powerful so the hiss is a bit more pronounced so they tend to keep them off when not in use. I couldn't hear it in a quiet room, so it's a bit subjective.
Genelec does recommend, that when you buy the monitors, you turn them on,
and you'll never turn them off ever again. This is basically to increase the lifetime of the components when avoiding unnecessary surges and to allow everything to settle so that they'll sound the same, every day, all day.
But anyway.
---Now a general rant about quality---KRK isn't very high end when it comes to studio monitors. Every decent monitor speaker can be bought one at a time, not as a pair. Genelec is bought one at a time because they can guarantee that any speaker coming from the production line is so close in tolerance that it can be paired up with any other from their production line without sacrificing anything.
My friend works at Genelec at this time, and he told me that every single speaker can be paired up with the serial number to the measurements done to it before packaging. And they do store eve3ry single measurement done to their speakers. Thus if I was so inclined, they could perhaps give me the measured frequency response for each of my speakers and I could in a sense use them as a decently calibrated sound sources for acoustical measurements.
That's quality control for you!