You need a response from someone more technical, but the "dual mass" was described to me as a "rubber doughnut" which stops noise, vibration etc. Many tuners have said that the DMF construction is at its limit at about 300bhp so if you have a KO4 and are going stage 2+ you need to swap to a SMF, the advantages being a more efficient design, the disadvantages being a knocking noise on idle and vibration into the cabin.
Stattlers did not concur with the DMF being knackered after 300bhp and put my original DMF back in with my Sachs and Wavetrac as there was nothing wrong with it. Certainly, 10,000 miles on and it is fine