If I remember, the ABus is the feature that allows you to set up the second zone with an IR module so that you can control the reciever from the second zone. I don't see that I will ever use this feature, but it is nice to have the option. So, does the HK do the A/B switching, or do I have to use the other two channels? This will not be a problem initailly, but if I do ever decide to go 6.1, then I will lose the amplification to the porch. Who knows, maybe by that time I'll be into seperates.