First, in terms of signal flow, the output is coming from the preamp and is going to the input of the amplifier. Second, I assume the preamp is SE and the amp is balanced. Cardas, and others, make high-quality XLR-RCA and RCA-XLR adapters but you might be better off using one cable with an XLR on one end and an RCA on the other. Most cable companies make such.
Kal
Kal