During my former job engineer specialized in airconditioning systems I have delt with complaints of crews and passengers smelling dirty socks or suffering from headaches. To be honest I (we) have never found a reasonable explaination (except for one occasion in which it was caused by the ground crew attachting a jet starter to the aircrafts extenal airconditioned air jack, and thereby blowing exhaust air from a small jet engine directly into the cabin). We have taken measuring equipement into the cabin, cleaned entire bleed air systems, replaced filters and had them chemically analysed, cleaned airco ducts but never really found a reason for the dirty sock smell. The next step that I was planning to do was to check the route of the airplane that it flew and see if maybe the smell was caused by the ingestion and compression of polluted air, however I left the company before I could start this part of the investigation.
And with regard to the fact that the problems are only emerging since the beginning of the 80-s, that because the aircraft manufacturers started to introduce re-circulation fans to decrease the amount of bleed air to be taken from the engine and thereby increasing the fuel efficiency of the aircraft but still meet the FAA/JAA required level of "fresh" air to each passenger. This lowered the number of times the air in the cabin was completly refreshed considerably. The filters in the recirc system cleaned the air from airborne particles making it "fresh" again, but since particles that cause air to smell are very small this was not filtered out. The latest HEPA filters are somewhat of an improvement since the mesh size of the filter is such that it filters out more airborne particles cleaning the air even more. However this does introduce a new problem and that is that the filters should be considered medical waste since it also filters out aerolized germs. When following the guidelines of the World Health Organization (WHO) in that case, maintenance personel should wear protective clothing and mouth and nose protection...
So you see when "solving" one thing you introduce another problem.
Cheers