Let me be the bad guy. I'm a well experienced Bose 901 guy. The Series VI are not that great even with the best equipment. With a 100 wpc mass market receiver, you are not even going to get sound as satisfying as a bluetooth boom box. Listen, this is where I started out but I had the much better Series I. I later tried the Series VI and I had to dump them. What you are hearing is what they are.
Sell the 901s and buy the Klipsch R600 towers or 800s if you can stretch. You'll then have something decent enough and your receiver will run them just fine. Whatever you do don't put any money into trying to get the Bose working better.