There is no need to use a hi-lo converter in the A4/S4. Your stock h/u has line outs (you just have to add RCA's or buy an adapter to add them).
If you have a Bose system, you can pickup the lineouts in the rear from the stock amp.
If you have the non-Bose system you can tap into the lineouts from the amp attached to the left rear speaker. The h/u also provides a remote amp on lead.
Checkout the B5 and the audio forums on
www.audiworld.com for instructions from where and how to run power for your amp to adding RCA and changing the stock audio components.
Good luck with the upgrade.