It's sad, Digidesign did not include a hardware mixer in the 003, you can find hundreds of old threads on DUC lamenting this.
To give you a flavor of what ADAT preamps are out there...
https://www.sweetwater.com/store/det...mic-mic-preamp (~$650 and will sound better than your 003).
https://www.sweetwater.com/store/det...audient-asp880 (very well regarded by many folks)
https://www.sweetwater.com/store/det...rophone-preamp (RME... drool)