When reading specs of speakers for frequency response, what is the difference between:

Freq Resp +/-3dB (Hz)
Freq Resp +3dB/-9dB(Hz)

Those specs specify the "margin of error" that the frequency measurements fall between. In other words, with the +3/-9 spec, the speaker is allowed to be as much as 3dB louder or 9dB softer than reference, while with the +3/-3dB spec the allowable range is 3dB louder and 3dB softer. You'll notice that the frequency range with the +3/-9dB spec is always larger (usually in the lower extension) than with the +3/-3dB spec. This is because the low end of speakers drop off more slowly than the high end.