HTfan,
I have had a Yamaha 5740 and I own Polk speakers (RTi8s, CSi5, FXi3s) - although I never tried to drive the Polks with the Yamaha - I used it for M22s.
You have a good receiver and good speakers. What the others have said may be right on. My Polk speakers are so good that shortly after I got them I was listening to a CD at what I thought were crystal clear, comfortably loud levels - only to find when I turned the music down that my pager, home phone (3 feet away) and cell phone (in my shirt pocket) had all gone off and I hadn't heard them.
With my 5740, I could get the music loud enough to irritate my wife, but not enough to really "rock the house" the way my home theater system (HK AVR 230 receiver and Velodyne CHT 12 sub added to the Polks) could - but I would have to turn the 5740 to -5 db or so to do it - in a huge room (8000 cubic feet).
So, with that, here are my thoughts:
It may be louder than you think - confirm with an SPL meter.
Go through the "set up" of the receiver again and see if it is set right - the "Night mode" or some other thing like it could be on.
There is a "stereo" setting on the receiver and it will put out a little more power per channel for 2 channels than 5. Plus, if you have it set for 5 channels it may be trying to send a big part of your sound to a center that doesn't exist. Also, make sure the fronts are set for "Large" so you are getting all the sound you are supposed to out of your 2 speakers.
If you add a really good center the volume will increase significantly in 5 channel mode.
Depending on how loud you want it you may need to take your receiver back and "upgrade" or add an amp if you really want it to blow the windows out of a big room. My HK AVR 230 is rated for fewer watts than the Yamaha 5740 was, but it is significantly more powerful.
