Background: 

You become aware, but you're not sure how, that a soft, low level hissssssing noise is being emitted by one of your speakers. You put your ear to the grille (or close) and, yes, there it is, a continuous low level hissssssing.  This can't be heard from the listening position, unless it's really quiet at night maybe. Anyhow, you've noticed it, and once noticed...well..

Scenario 1 :  This will be a leak from a bass panel to ground/earth, somewhere along the perimeter of the frame.  Check for a soft purple glow somewhere (dark room needed).  The is caused by 6kV bleeding off and creating a corona discharge as it does so. Do you need to "fix" it?  Depends on how much it annoys you.  The speaker has to be disassembled and the edges of the bass panel re-insulated with a good quality tape, preferably silicone. So, does it annoy you that much?

Scenario 2: The EHT unit is discharging somewhere, usually the 6kV terminal. This can even be caused by a sharp point in the soldering or a protruding wire thread from sloppy soldering work.  Solder joints need to be perfectly round.  Any "pointy bits" will start "spraying" electricity into the air.  Again, a corona discharge will be seen in a dark room as a purple glow.  This should be fixed.  Check also for any nearby structure or wire that could be leaking.