I have 2 REL Stadium subs and Al's explanation is correct from my experience of them. You have nothing to worry about.
8 Ohm speaker with 4 ohm subwoofer?
I run Spendor s3/5r's using the 8 ohm posts on a Jolida tube and have just purchased a Rel q108e which according to the Rel specs runs at 4 ohms. I planned on running the subwoofer directly to the same 8 ohm speaker level inputs on the amp, but now I am worried that may not work because of the ohm difference. Please help so I can decide if I need to cancel the order quick.