-6dB means the same thing as +/-3dB. +/-3dB isn't really a standard of any kind, its just a frequently used window. It isn't all that useful since it allows 6dB swings in response anyway.
The nominal response isn't really worse than that of the SVS subwoofer. it doesn't remain as flat down to low frequencies as the SVS, since it places more emphasis on 25Hz+ range. But in its extended mode, its not really that far off from the SVS 2000 subs. Here is a comparison of the PC-2000 nominal response vs the 12" THX Ultra nominal response in extended EQ mode with one port sealed (purple curve is PC-2000, green curve is Monolith 12):
In terms of extension, they are about the same, with maybe a tiny edge going to the 12" THX Ultra. They just have somewhat different response shapes. The SVS sub places a bit more emphasis on deep bass. I wouldn't say that one is better than the other, they are just different target curves. Remember this is nominal response and not maximum output response.