I have a Yamaha RX-V557.
I have a "Direct Stereo" mode, which completely bypasses the sub, all mode comes through.
It looks like they renamed it from "Pure Direct" to "Direct Stereo"
Pure direct is what you want. No need to turn off the sub, because the reciever will send nothing to it.
On yamaha recievers there is also a straight mode, which is what I use all the time. What this does is bypass any field processing, which means the receiver won't process speaker distances, room size, or anything like that, but the sub will be used. Some processing is still done to turn the sub on. Direct stereo (in your case pure direct) completely skips everything, true and true stereo only processing what it needs to get a stereo signal.
