I would use whichever unit has the best crossover. My H/U has amazing crossover capability so I run my amps on full mode and use my H/U to cross it.

If your amps have better x-overs than the head then by all means use your amp.

The setpoint you use is entirely setup dependent. I have mine set at 60hz with a 12db/octave slope but only because my mids can handle going that low. I have my sub set at 100hz with a 24db/octave slope. I have them set this way so they blend seamlessly no matter what you are listening to.

You really just need to play around with it and use what sounds best to you.

 

DONE! Just got back from outside. Took me about 1hour to test and tweak things around and try different setting. I ended up:

1. Turning off the HU HP filter. It's not very good.

2. Turning off the HP filter on the amp speaker amp and let the crossovers handle the frequencies. It seems to sound a just a tad bit better than if I enable the filter on the amp and set it to 60Hz. I imagine Polk has the crossover set optimally for the speaker from factory so I am falling back on this assumption.

3. Set the LP filter on the sub to 75Hz. Slightly turn the gain up since my heavy metal doesn't seem t have much down low. This gives me better fill in base and good, tight response when I hit double base in the music. I turned down the Base EQ on the amp as it was causing too much of the drowning base when I was listening at my usual listening volume. This combination seems to sound best overall.

Since I turned up the gain on my sub, I was concerned with overdriving the speaker. I took some measurements while playing music and I wasn't coming remotely close to my peak, safe output that I was able to reach with the 40Hz test tone. Is this typical or is just related to the type of music that I listen to?

BTW, the system sounds AWESOME now. It went from ble to amazing, just by tweaking the right settings and getting the right power to the speakers. Fantastic!!!