HFTs are the little concave discs that attach to the walls at reflection points, they come in sets of five for $295, ideally you use them in conjunction with an energizer either the FEQ or Atmosphere
more about these here
https://forum.audiogon.com/discussions/synergistic-research-hft-s?highlight=Synergistic%2BHFTs%2B
more about these here
https://forum.audiogon.com/discussions/synergistic-research-hft-s?highlight=Synergistic%2BHFTs%2B