Look into the Cerwin-Vega CLS-215 or CLS-15. The 215 has been very favorably reviewed (surprisingly) by both The Absolute Sound and Soundstage. They're about 95 dB efficient and are pretty flat down to 26 hz. They are also very flat in amplitude; the response curve at 90 dB is nearly identical at 95 and 100 dB.
Available within your budget at Amazon.
Available within your budget at Amazon.