All the caps in the ESL portion are already film caps, so no refurb is possible there. There is an electrolytic cap parallel to the woofer, but it won't affect your top end.
So, yes, it could be that your panels are so dirty that they are either not holding HV charge, or the charge is bleeding off via a dirty path.
With my ESLs the main enemy is spider droppings and exploded insects, and this past month when bugs move inside there can be new 'splats' every day. I get out my magnifiers, special soft brush, and vacuum very thoroughly, esp at the top surface of the spars where dirt settles.
But there can be other airborne contaminants like cooking fumes, tobacco smoke, ocean humidity, etc that can build up and cause the panel to not hold charge, and that is probably the fate of every ESL in the long term.
You should note that showering them is a LAST resort and that the panel sandwich is vulnerable to separation and stator movement, so keep them held together if you try.
Are they both faded in the top end by the same level? Try swapping them L to R to see if one is noticeably worse, which might indicate something other than normal dirt build-up.
All for now!
