"Discouraging answer" indeed! Another way of saying it is "In the airplane, with so much other noise, that little hiss will be masked out". And they're correct. And they have a bunch of bad circuit boards out in our hands. I have a set of LS Zulu's, and recently it developed a low-level zzzzzz in one side. (BTW, zzzzz is not exactly the same as sssss, but similar as far as electronic failure is concerned. Neither are the same as a low-frequency "hmmmm", which is a grounding issue.)

Your speakers are not creating the noise, just telling you the bad news. The bad news is: modern small circuit boards are built with micro-components, that are not replaceable by any but the most well-equipped shops. And that R&R cost is completely prohibitive for this level of product. Lightspeed would just replace the whole electronic unit, if they still stock them.

Since I can't hear the problem in cruise, I'm just living with it. Problem is unlikely to get worse until the unit goes out altogether.