[archtop]

Digi plug doesn't have an output level control,
only input(as you noticed),
and yes you are able to achieve more gain with output of stand alone compressor cranked.
But sometimes louder is percieved as better,wich might not be true if test was done with equal volume/loudness.
A good way to use your output level is to have it match the uncompressed level(bypass)
unless of course it's a live gig with an underpowered P.A.
and you need extra gain.