Just to be clear this is what your constraints are;
300-600 pounds (or $600 - $1200)
Frequency range of
at least 60Hz - 20kHz
Good dispersion
Good imaging
No size constraint
Reference levels at 4 meters (so peak output of at least 100 - 110 dB @ 1 meter)
That's a healthy budget for two speakers so this is probably do-able. I'd start off by browsing existing designs. Other then browsing this forum check out these sites and see if anything grabs you;
GR Research Zaph Audio
Uh, I'd post more but I have to go now. I'm sure others will chime in.