Photo multiplier tube and lock in amplifier
You will be able to set up the Stanford lock in to use it's own amplifier (in which case you should select current input) or use the separate amplifier you have (in which case select voltage input). Compare the specifications of the built-in and separate amplifiers, and choose the one with the enough gain and the least noise in the frequency band you're interested in. We can't recommend one way or the other unless you give us the specification of both amplifiers.