check your output settings
I have an HK AVR 630. When I first set mine up I felt the same way as you. Then I realized that I was playing different surround modes without setting up each one individually. You have to go into the setup and adjust these to get a balanced output based on your listening position using a SPL (sound pressure level) meter. I boosted the output anywhere from +4 to +10 dB per channel and the system will now rock the house down if I want.
As to a using a sub, they are great for home theater. Since I have them built into my speakers (Def Tech's) I use them for music as well, but usually reduce the output level. You didn't say how you use your system (or I missed it) but if it's for music only I'm not sure I'd want to add a sub. It would depend on the type of music you listen to. Your B & W's should be more than adequate by themselves for great music in such a small space, especially with hardwood floors.
Good luck and let us know how it turns out.
