I have removed one from a Sub box and replaced with another amp. The only specs I know is the amp turn on voltage from a Ford/Blau HU is around 4.6 volts and only activates only if there is a musical signal present. It very light weight and flimsey. Don't know if that helps.
There is no amplifier on the front and rear speakers. The Sub has the only amp on the system.
I agree that the amp turn on voltage is around 4.5 volts DC. I just recently replaced my Audiophile head unit with a Kenwood KDC-X679. I also integrated the factory sub with the Kenwood's sub output. The car sounds dramatically better using the adjustable crossovers in the X679. It is a little louder and the bass does not deminish in direct proportion to the volume level.
I think the bass diminishes as you turn it up on the factory HU's to eliminate clipping in the amps. It should only happen when the output power approaches its limit. The sub out shouldn't be affected since it is low-level. My Focus/Blau MP3 HU has the same symptoms when I turn it up. The volume indicator goes up to 66 but after about 56, it doesn't get much louder. It just starts compressing and limiting the musical peaks. You can fool it for a split second by turning up the volume quickly to the max 66. The sound gets very loud briefly and then comes back down to the limited maximum. I try to avoid playing music at 66 because it is not as dynamic as if it was set to about 56 or 60.