I ‘d never own a subWofer so I may not be the best person to answer your question...
You probably need to send the analog signal of the 2 front channels to your sub amp. I believe most subs have extra inputs for 2 channels setups.
Dont send the digital signal to your receiver 'cus it'll not be true SACD or dvd-a audio it'll be either pcm ,dd51 or dts.

Jaime


The sailor does not pray for wind, he learns to sail. --Lindborg