As a matter of technical interest to anyone else looking at the forum, one of the main reasons why these old optical compressors of mine sound so good is that during compression the 2nd order distortion (even harmonics) rises slightly, which has the psychoacoustic effect of making the brain think that the sound is louder than it really is. This is completely different to all other types of compressor where distortions stay the same or on some units, the 3rd order (unmusical) distortions rise; and they sound horrible! That, in conjunction with the shape of the attack and release curves, makes the compressor sound smooth and warm, even under severe (unrealistic) compression. 
It sounds as though I’m pretending to be a Hi Fi anorak when I say that there are many factors in the operation of the optical compressor that are difficult or impossible to measure, mainly because they are transient effects of non-linearity not seen by the best measurement techniques, and certainly not reproduced with digital plug-ins!!