Hi Robert, thank you for your reply. I was aware of this possibility, which is why I went for the newer models. Although the serial number decoder doesn't include the EM range , based on another member's post my Xs appear to have been made in Nov 2019, (ending L11) and the cabinet numbers match the panel numbers. Also they are in excellent condition, no sign of dirt or dust. So I am surprised, it's fairly subtle, if I didn't have the comparison I may not have noticed, but I guess it can't be anything else. The ESLs are probably of a similar year, as they were only a couple of years old when I purchased them and they are fine.
You don't get slightly better top end with smaller panels?
I have compensated with a software eq for now, but it often glitches. I'll experiment with Audyssey too, but that seems to be asking for a network based AVR before it will do anything.