I want to modify my sound with VST before sending it to my amplifier.
My sound card is Focusrite Scarlett 2i4 and my amp is Ampeg BA108.
So it will look like this: Bass -> Sound card -> Amp
My question is what input on my amp should i use and what cable do i need?

Amp inputs:
Effect Loop RETURN & SEND
CD Input
Instrument Jack input

Sound card outputs:
RCA