Well I have had a similar problem in my new listening room. Much more space from side wall to listening position due to long wall placement, and a vaulted ceiling that peaks right down the middle of the room. I was having more sound issues than I ever hand to deal with before. Two posts on this thread are leading you in the right direction IMO. More efficient speakers and horns.
Let me further add this. In addition to higher efficiency and a horn/waveguide design, look for a speaker that is more flexible as to placement within the room and has the ability to allow for user tuning. My speakers are Audiokinesis Jazz Modules and I use them with amps ranging from 10 watts to 70 watts. They are 92 dB efficient, use a 90 degree waveguide, and allow for both bass (via adjustable port length) and high frequency (via external resistor swapping) tuning. In my room, the bass tuning capability solved my problem immediately. In your case you will find these speakers will load your room nicely with the amps you currently use. You will also be pleased with the dynamics and natural timbre of the sound, especially on classical music.
At 92+ dB efficiency you'll have many choices out there and I admit I'm a bit biased here, but Johnk's and Duke's advice shouldn't be ignored.
