There shouldn't be any double bass issue. Typically, when you set your speakers to "large" it means that they are run full range and no crossover is applied. Because you have deemed those speakers as full-range the bass from the left and right channels will not be folded in to the subwoofer. So, no double bass. The subwoofer will be playing the LFE channel and the bass from the center speaker and surrounds (because they are labeled "small")
Some receivers have a "double bass" option which allows you to run the left and right channels full range plus have their bass folded in to the subwoofer, but the "double bass" has to be enabled for this to happen.
I'm surprised that your receiver doesn't allow for independent crossover frequencies...what's model is it?