Output levels.

I am looking for a bit of help on a monitoring circuit.
Currently for setup puposes, I am using a CD transport going through M-Audio converter It supplies signal for sub. I have the sound I am looking for but would like to use ART DIO and a passive volume control I would like to split the signal for sub. The pass through on sub are not detailed . Some one said to use Y-cord. Will this affect levels? between sub and Mains. I want the transparent and practical solution
Thanks