If they are (and many are not) that's what a good sub is for on the rare occasion when anything other than ambient or effect sound is directed to the surrounds. I doubt the OP would ever notice the difference between the RB 41 or 51.
Depending on his room layout he might be better off with one of the RS models, as long as multi-channel music is not a priority.
there are many here on the thread who would agree that surround channels DO have the same amount of content & signal as the mains , the only way to actually experience it is to have the surrounds be able to keep up with the mains by matching the horn size & to keep up with the mains it is important to have the same size of the horn as in your mains , That is why the RB OR RS speakers are great with a matching horn size RF speaker as a main .
Yes I would agree that "maybe" the RS will work better but that is room shape dependent , direct radiators (RB-XX speakers ) work just great if you are able to aim the horns @ the listening position . BUT the RS surround speakers come at a high cost over the RB direct radiator of the same size horn as a RS .