What kind of speaker wire are you using? Did you get some 16 gauge oxygen-free copper or thicker to go with your new speakers? Or did you reuse the the stuff that came with your home theater in a box? I have a suspicion your HTIB came with 22 gauge disposable stuff; and if you're still using it, that could be the cause of the treachery.
See this page for a thorough explanation, and be advised that copper-clad aluminum wire (like Amazon Basics) is only about 2/3 as efficient as oxygen-free copper (like Monoprice or Belden).