If I were to wire my amp to my the FF4 though the speaker level inputs then out to my speakers through the speaker level outputs, is there any change or attenuation to the signal that my speakers get vs just wiring my speakers directly through my amp?

If I were to play a frequency above the lowpass crossover would it be the same volume without distortion as if I had wired the speakers directly up to the amp or is the FF4 doing anything to the signal?

The speaker level outputs have a 6 dB per octave high pass filter, for use with SuperSats, at around 150 Hz. (Info is in owners manual on this hookup.)

Not sure what you are asking in the second question (distortion? the signal gets attenuated above a low-pass crossover, hence the name.), but I'm pretty sure the answer you are looking for is yes. The frequencies above the high-pass crossover in the sub's speaker level input/output section are sent to the main speakers unaffected.