I am sure you have already had this answer, but incase you haven't. Using rca's means your Signal will be clearer, because rca's don't have the resistance associated with speaker line inputs which use your amps wattage as a powered source. And every watt counts when it comes to the clarity. Plus not needing the extra draw on the reciever or amp will free it up for your other speakers. Less stress always helps.
Now to confuse you a bit. When used in any other setup than stereo I've found using the speaker line inputs and having the sensitivity on low seems to match the rest of the icon series a bit better.
In the end of it all, it will sound good both ways as long as you tune them and make the adjustment needed. Keep them far from walls. At the very least 18-24" off the wall helps dramatically. I've got mine in my bedroom and it limits how far I can pull them off but I have the face around 20"
Remember experiment and have fun.