The receiver is only checking sound levels though and doesn't know about the voltage requirements the subs may have to keep them on. So either way, on the receiver side or the sub side, the voltage needs to be raised. Approximately what % are they set on the subs? 50% volume or lower? If they are set lower than 50% now, I'd adjust their levels up and lower the receiver side. If they are set higher, I'd drop them down a bit more and raise the receiver's level.