I think the confusion is that most of these guys seem to think you have a newer digital receiver. Your original Post mentions that you have an older Yamaha ProLogic receiver. There is no setting for Large or Small for you main speakers. Disregard this advice. Bottom line is if you have an active sub(meaning it has its own amp) you will need to make sure it has speaker level inputs. If so, I would suggest going from your 'A' speaker connection, to the sub in, and then out to your main speakers. In many cases the speaker level outputs from the sub are crossed over, cutting out all info below 100hz or so. This will help your mains operate more efficiently (especially if they have small bass drivers). It is best not to use your 'B' speakers as this will generally place a heavier load on your amplifier.

Hope this helps...