I beg your pardon but I doubt it if the problem is in the input section of the sub for any decent sub contains redundant/cascaded lpf in it's frequency filter section also located in it's input. Now if for example the rf noise is being received in the sub's input via porous rf transmissions then it should have been filtered out.
The only way for rf, am/fm or whatever radio signals to pass is if it is forcedly injected as an input signal, which leads me to suspect the AVR.
Although I'm not discounting the fact that the AVR + Ixos cable + REL sub accidentally forms an RF receiver, anything is possible.
Lancito,
If the rf problem is local to your AVR's sub out, one way of really checking this is to use the AVR's sub out and connect it to an integrated amp with speakers (even one channel only), this way we can be sure if the problem is really from the AVR or not...