Hi Ascension,
I haven't had my EP500 opened up for a couple of years, but if memory serves, there is also a plastic boot around the amplifier inside the EP500, and the boot can rattle, so you should check it as well.
When we were developing the EP500, I exchanged about three different amplifiers, as well as different boards on the amplifier, so I had to open it up numerous times. I seem to remember a rattle developing with the plastic boot, and I managed to get rid of it.
Also keep in mind that every object in your room has its own resonant frequency that can be set into motion by a particular frequency. I once had metal Venetian blinds that rattled and I swore it was coming from the subwoofer.
Regards,
Alan
