Brooks Berdan may have mounted more ET arms on turntables than anyone else in the world. His original choice for the arm was the Oracle Delphi, for which he created a mod eventually incorporated into the table by Oracle itself (a round block of stainless steel added onto the bottom of the floating subchassis at a specific location, to make its’ mass perfectly distributed and therefore more dynamically balanced. Brooks had training in and knowledge of the design of race car suspensions).
He added the VPI HW-19 when that table was introduced, and found its stiffer-suspension (and to a lesser degree its higher-mass floating subchassis) to provide a more stable platform for the unusual moving mass of the ET. The reason for that is that the center of the arms mass changes location as the arm moves across the LP more than does that of most non-linear arms (the exception being very high mass arms of that sort); a table with a softer suspension can have it’s floating subchassis become slightly out-of-level, while a stiffer suspension will be less effected by the changing location of the arms mass.
Of course, a table with no "normal" suspension (metal springs, air---the Townshend Seismic Sink, Sorbothane or Navcom) will be completely unaffected by the moving mass of the ET. Brooks mounted the ET on a lot of VPI TNT’s.