This thread is strange. I don't remember which Onkyo you tried, but I do recall most folks suggested you check your M80s before grabbing a new amplifier. Of course, it's your money.

Now you notice that the M80s need a subwoofer, and I think you're right. However, as ClubNeon points out, a small sub will not work. You need a sub to fill 20Hz-50Hz. That's not a small sub.

You need a powerful sub, which puts out clean low frequency output. I'd consider a Hsu, Epik Legend, or Empire, if you're on a budget, or one of the big Velodynes or Axioms if not.

Enjoy the Music. Trust your ears. Laugh at Folks Who Claim to Know it All.