The receiver would have to convert all 6 analog input channels simultaneously to digital, so that bass management can be applied. Not a feature enough users care about to warrant the cost increase to the reciever manufacturers. As John mentioned, there are some receivers that will do this- the Lexicon RV-8 I have does, and I love it. No worries or settings to change for different sources, but it certainly wasn't an inexpensive investment.
Also as John mentioned, turning on the bass management in your SACD player (setting speakers to small) should let it re-route the bass to your subwoofer channel before it leaves the player.
