I have noticed that side-chain inputs are not common on the more traditional compressor designs. I understand why they would not exist on an exacting "reissue" model, like the "new" UA LA-2A & 1176 models - if the original model didn't have the feature, the reissue won't have it either. But can anyone explain why non-reissues like the Manley, TubeTech, Mercury 66, EAR, Requisite Audio and other "classic style" compressors (both optical and vari-mu) typically do not offer side-chain inputs? I'm guessing that it's not a cost saving issue, because these are all $2500 to $5500 units. Is it technically difficult to incorporate a side chain circuit into optical or Vari-mu designs? My reason for asking is that I am looking at several of these models for future purchase, and always thougtht that a side chain input would be a desirable feature ( and a feature that NONE of my existing comp's offer). Why do most all of the $500 compressors in the guitar store audio counter offer side chain circuits and seemingly so few of the high end models? Are there negative aspects to this type of feature that make it less desirable to some designers?