It sounds like an impedance problem . MTM design speakers which utilize 6.5 in drivers commonly have sub 4 ohm loads often 3 ohm or less . The Dennon and Yamaha are said to be 4 ohm stable at best but it varies some what .
Most of the DIY speaker builders I speak with will tell you that MTM speakers that size can be difficult to drive . Stepping into a mono block amp will give you your best chance at solving your problems . Three M200's across the front would seem ideal for your setup . You could utilize the Dennon or Yamaha as a temp. pre/pro . to balance things out until you were able to upgrade .