Why is the Doubler output level about 1 db lower than the input signal?
We reduce the gain slightly to allow head room as we process the signals - part of the process creates a small gain. Reducing the overall gain by 1 db avoids any possibility of the signal clipping.
The stereo output double always seems a little left heavy?
This will depend on the exact settings, as of the purpose of the Doubler is to create different signals. We do not really create a stereo double pair but two output signals which can be used in the mix.