I recommend installing one of these Auralex Acoustics SubDude Subwoofer Acoustic Isolation Platform underneath your sub. Very effective in reducing floor borne vibrations and helps tighten the bass.
https://www.amazon.com/gp/product/B00COVEJ1E/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1
https://www.amazon.com/gp/product/B00COVEJ1E/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1