Hooking up MARANTZ MODEL 15 without a PREAMP?

I bought this beautiful Marantz Model 15, which is vintage…which contains two mono blocks uniquely built into one unit… with only speaker wires OUT, on each mono block… I haven’t anything preamp integrated to feed it into my Bluesound node! What a stupid move on my part! What do I do now as my Cornwall IVs wait for me to fix this dilemma! Does anyone know how to hook up these two mono blocks to my Cornwall fours and integrate the Bluesound node? I feel so stupid not to realize and purchased this very first Marantz solid-state amplifier from 1966…GEEZ