In reply to:
tRio has a "line out" setting under its "settings" menu allowing you to choose between either "fixed" or "variable" line-out volume.
In "fixed" mode, the line-out volume is fixed, but the amplified and headphone output can be adjusted via the wheel on the front panel or volume buttons on the remote.
In "variable" mode, all the outputs are adjusted via the wheel and remote.
Thanks for the good description of tRio's volume control capabilities.
Now I know that it'll work for me without having to switch to it right now in order to determine that fact.
I do plan to switch later, but now one of the most important questions for me regarding tRio has been answered.