Good news is you have powered speakers(?), right? Your receivers will power these better than non-powereds, and you should be good on dynamics and power overall, all things considdered. Separates would still be much much better in ALL AREAS HOWEVER! I'd go there.
Still, if you must do a receiver, I'd upgrade to something better. The Yamaha IS NOT a good match with the Def Tech's IMO. I've sold Deftech in 2 stores now, and Yamaha in 4.
If you need current receivers with all the lattest, I'd stick with Denon, NAD(build quality????????), Marantz, Nakamichi, B&K, Arcam, and other more full bodied, even forgiving receivers.
I'd keep my eye out for what's on the used market if I was looking for a receiver.
Maybe used Arcam 100, Bigger Marantz/Denon. Depends on your price range however.
If it was my money? I'd sell the Yam'ie and go for a used separates. Lots to chose from, and better sound for your money.
Infact, you can easily get into sub $1k mark with amp and pre if you play it right!
Case in point, there's at least two Outlaw 950 pre's under $500, and you could find a used 100x7(excellent amp) for that price if you looked as well! Otherwise, Parasound HCA806 6 channel amp's pop up now and then for $450 range.
Any other number of 6 channel pre's are out there in the budget range if you look as well.
However, the above receivers won't steer you wrong as well.
I'd still get the best receiver you can get for you money used however!(or at least a "new" deal on line, but beware)
The more expensive/bigger the receiver, the better off you'll be sonically. good luck