Isn't that rather a bold, blanket statement given the lack of details known about the system is question?... Having more power from a dedicated amp on just the main channels will dramatically help with this and the sound will improve.
If he has efficient speakers (as indicated in his post), especially in a small room, his receiver may be more than adequate to drive them to high enough levels w/o clipping.
Sure, if you're driving ~85db @1W/1m speakers or <=4 ohm speakers or trying to fill a large room with sound with just a receiver, a dedicated amp (esp for the LCR speakers) almost certainly will help, but in some (perhaps many?) cases it's not necessary.
Not trying to be argumentative or contrary, just saying we should see details before indicating a dedicated amp will make a "dramatic" difference.