Originally Posted by Bill Fitzmaurice
You can only get so much out of a pair of 5 inch drivers. You've found out how much. Big sound requires big speakers, that's something you can't get around.
Bill makes a good point. You could be exceeding the ability of the speaker to perform at the volumes you are looking for. One way to test this theory is to disconnect the other speakers (being sure the bare wires do not touch) allowing the receiver to push more power to the remaining speaker. Turn the volume up to the level you normally run at and replay the scene that caused the issue. Does it sound OK now? If so, it's a wattage problem, If not, you have the issue Bill mentioned, or you have a blown speaker.
As to the receiver power issue, that can be a difficult one to answer. There is a lot of "fudging" of power numbers among receiver manufacturers. The Yamahas are known to be one of the worst in this regard. Your actual available power, all channels driven, is probably more in the neighborhood of 30 watts per channel.
FYI: Receivers that are known for not fudging the numbers are Denon, Marantz, and NAD. A fifty watt receiver from any of these brands will clearly outperform the 90 watt rated Yamaha.
One last point, bass takes a lot more power than mids and treble. Are you crossing over the speakers to a sub? At what frequency? If you set the center channel to small, crossing over at 80 or 100 Hz, you will need far less power to drive the speaker to the level you want. The trade-off will be less bass from the center channel (that should be the sub's job anyway).
Let us know the results of your testing.