I'd say an actual amp would have been a better choice than the Yamaha. As I mentioned already, it is probably about the same amount of power as your receiver; definitely not enough to be a big benefit over the Onkyo.
If you aren't hearing what may be distortion at the levels you are listening at, meaning if it sounds less and less clear as you turn it up, then more power would help. If it sounds good at the level you listen at and you don't need to turn it up beyond that, then you should be fine.
I'd try it with the Onkyo by itself just to compare. It kind of sounds like whoever sold you the setup either undersold you (not enough amp, receiver) or oversold you (amp you didn't really need), because it is kind of dumb to sell you something that doesn't have enough power for what you want to do in the first place. A good shop would have asked the questions I asked earlier.