I am adding a sub and separate amp to my F90. My existing amp has an RCA output they recommend as a pass through for a sub amp. So should I run an RCA from the HU to the sub amp or should I just jump from amp to amp. Also, any advice on an external crossover vs using the HPF and crossover on the HU?

Thanks and sorry if I am asking something that's been answered, I did search first.

I'd use the sub out on the head unit. You'll have much more flexibility that way, since you can adjust the level, crossover frequency, and even turn the sub off, right from your dashboard. You'd lose all of those abilities if you used the pass-through on your other amp.

as mentioned above, run the RCA and do it right so that you have control over the system. Pass throughs are for people putting in systems and there radios do not have enough outputs for.

As for the crossovers it depends on which piece of equiptment has the better crossover, when i had my alpine headunit, the alpine had 24 Db/Octave (decibals per octave)(the higher the # the better) and my amps had 18 Db/Octave, so i used the crossovers in the headunit. now that i switched to a pioneer headunit with a 12 Db/Octave F+R and 18 Db/Octave subs and kicker amps with a 36 Db/octave, i use the much better crossovers in the amps