The room youre in is quite large. So this is not surprising.
All B&W's are not as efficient as they look on paper. Except for maybe the 300 series.
Try using the pre-outs on the Yamaha and get a bigger amp for the speakers. Lower end Yamahas dont have the strongest amp sections. The B&W's like alot of current to sound there best and lower end receivers can't deliver this. Especially if youre trying to fill a larger room with sound that has some impact.
Alot of people use HT receivers with subs and cross things over at around 80hz which helps the amp section sound a good bit stronger. When you run speakers full range you cause the reciever to become less efficient since its having to amplify the bass section as well. Which sucks up most of the power.
More power should take care of this. Plus a powered sub would not hurt to fill things out and give things a fuller sound in this large room your in.
Cheers. Hope this helps.