That's the conclusion I've come to. There doesn't seem to be an alternative and make-up gain does solve the problem. Strange though.
 
Thanks.

You said it's worse without the plugin. So the problem is not happening in the plugin, it's happening in the output stage of the bus which, again, leads me to suspect offset mode.

You're right, it is worse without the plug - but I don't know what offset mode is.

If you typed "O" you put Sonar in Offset mode. Below is from the manual.
 
Envelope mode and Offset mode
There are two modes which control how your volume faders, pan faders, bus send faders, and bus send pan
faders behave during playback. The two modes are Envelope mode and Offset mode.
Envelope mode. In envelope mode, volume and pan faders follow the project’s automation and do not respond
to changes you make in real-time.
Offset mode. In Offset mode, you “offset” the current automation in a track using a parameter’s controls. For
example, if a pan envelope is set to hard left (100% left) and you adjust the pan in offset mode to 100% right, then
the pan parameter is now set to hard right. Setting the pan in offset mode to 50% right would set the pan to the
center.
Note: Any position that you set a fader to in Offset mode remains in effect when you switch back to Envelope
mode. For example, if you set a volume fader to -INF while in Offset mode, switch to Envelope mode and
drag the fader to its maximum level, you will not hear anything.

And OFFSET is useful how? Just trying to understand why this tool exists.  I can't tell you how many times I had OFFSET mode on and didn't realize it.  Usually took some extreme situation that mixing moves didn't have the result that I anticipated.  Oh, the "O" was accidentally pressed somewhere along the line.
 
Jim

Offset is extremely useful if you have a track that's been heavily automated with a complex envelope but you now want to raise or lower the level of the whole track by a given amount.
 
Engage offset mode (all faders will jump to zero, over-riding the automation), make the change(s) you want on your selected track and then disengage offset mode. The faders will now snap back to where they were previously.