While Opto compressors use an electro-optical attenuator, FETs uses  voltage to control the flow of current. With lightning-fast reaction times, FET work great with  snappy transients, providing character to your drums and guitars. With an innate delayed response to light, Opto compressors are best for musically smoothing vocals and mix busses. But since the Opto compressor has an initial fast response on top of which the delayed releases are built,  it provides a narrow window where the heavy transient materials are heard. But since the upcoming releases don't clamp down as fast as a FET, you don't get FET-style distortion and character. That being said, FET and Opto compressors complement each other very well. Setting up the FET  1176 in front of the Opto compressor LA 2A has been practised for decades. Unlike the two-knob minimalist LA 2A, the FET 1176 has attack, release, and four ratio knobs along with input and output. While the FET1176 doesn't have a threshold, cranking up the input gain has the effect of lowering the threshold.  The LA2A works with a peak reduction knob that replaces the attack release and ratio controls. This knob compresses audio by lowering the threshold, and the tube-based output gain compensates for the reduced levels.