Adding speaker protection for Adcom GFA-645


I have recently put together a tri-amp setup with an Adcom GFA-645 for the midrange. It works great, but I am a bit worried about the fact that the Adcom does not have any kind of speaker protection built into the amp.

I am fairly new to DIY electronics... so I don't exactly understand what I could do to prevent my midrange speakers from blowing. The manual of my active crossover (Pioneer D-23) recommends to add a capacitor between amp and driver, and provides a schematic for calculating the correct capacitor.

Is that all I need? And, does that in any way degrade the sound/frequency response? Somewhere else I read that I just need to add a resistor, is that another valid approach?