Do you mean with the back or side against the wall?
By against the wall do you mean flat on the wall or 6" away.
White makes it tough.
I'm thinking you might need sealed or, at least, front ported.
Sealed options:
Emotiva ERM-1 (but it is only black).
Aperion Intimus 4B (but only cherry or black)
Front ported:
Usher S520 (in white!)
Wharfdale Diamond 9.1 (Cherry - it is a pretty light shade)
Scroll down here to see wood-tone:
http://www.stereophile.com/budgetcomponents/1105wharfedale/
I have the Wharfdales set up as an impromptu stereo in my livingroom, and it really does a nice job filling the room (but it is not quite as large as your - ~18' X 27' with vaulted ceiling).
Cheers,
Kurt