On the back panel of the 1818VSL you'll find your ADAT I/O ports.
ADAT–S/MUX Input and Output. These are ADAT–S/MUX connections for your external digital devices. When recording or playing back at 44.1 kHz or 48 kHz, the ADAT input and output will stream 8 channels. When recording or playing back at 88.2 kHz or 96 kHz, each connection will stream 4 channels. The ADAT–S/MUX I/O also allows the AudioBox to send and receive word clock to external digital devices. Power User Tip: Remember to select “ADAT ” as the Clock Source and set the sample rate to correspond the external device in the AudioBox VSL Setup Tab (Windows) or in Audio MIDI Setup (OS X) when using the ADAT input for external sync
Taken from Page 8 in the AudioBox 1818VSL Owners Manual
PreSonus sells the following 8 channel preamps:
DigiMax D8 and Digimax DP88 (link)
