Well, the 990's bass management is flexible enough to work with nearly any low frequency response -3dB point, and the high frequency ought to at least reach 20,000Hz with any decent speaker (since that's where human hearing ends even in the best cases). The efficiency (which will likely range from 85dB to 93dB, with some horn speakers breaking 100dB) is going to relate solely to the amplifier. Seriously, find the speaker that sounds best to you, find an amp that can drive it, and the 990 will be happy to do its job with whatever you have.