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ah my alpines have adjustable slopes. 12dB should be fine though.

Yeah, I know the less expensive Alpines are limited to 12 dB/octave slopes as well. Regardless, play with it, but it probably will sound best with lowpass @ 80 Hz and highpass at 125. I had an old Soundstream DX5 Xover that had a 24 dB/oct lowpass and a 6 dB/oct highpass. My A/D/S 5 1/4's blended real well with my 10w5's with the highpass at 150 and lowpass at 80.

-Fishy

[quote="Harrison486"]I wonder if I can adjust the slope on my HU[/quote]
i think its mostly the high end models that do that. My pioneer 5500 did not do that, it just had the crossover, but my 8600 does have that feature.
(thats all I know as far as pioneer)

I wonder if I can adjust the slope on my HU

read this
[url]http://www.bcae1.com/xovrslop.htm[/url]

crossovers aren't brick-wall filters. the frequency you set the XO for is the point where the volume will begin to gradually decrease as the frequency rises or lowers (LPF or HPF). Use the head unit crossovers for everything. you should be able to set the slope or "Q" as well as XO frequency.
I'd set LPF to 80Hz 18dB slope, and HPF to 125Hz 12dB slope

a 12dB slope means that every octave (half or double the frequency) the volume will decrease by 12dB, or approximately 1/4 the volume.
so 80Hz LPF 18dB slope means at 80Hz it's full volume. at 160Hz, the volume is -18dB quieter. At 320Hz, the volume drops another -18dB..
that's why you leave a gap. both the midrange and the sub will gradually fade off in volume as the frequency changes, so the two "slopes" will meet up and fill in that gap.
nothing lost.

I'd just set the lowpass at 80 and try different highpass settings til it sounds good. Depending on the speaker/mounting your mids may already have a frequency response that rolls off substantially to 80 hz. The nice thing about going with 125 Hz or higher is that it will put slightly less demands on your deck's amp so it may sound better at higher volumes.

Using both lowpass filters will give you a steeper rolloff. I'd set the amp at 80 and again experiment with the deck's lowpass filter. If you limit the lowpass frequency for both to 80 Hz there's not really that many different combinations to try. It should be pretty easy to figure out which sounds the best and thats really all that counts.

-Fishy

When you set a crossover it just does not magically hold all frequencies beyond that point. It actually fades them out. Dependeant on the type of crossover it is this amount can vary.

So roll off is this area where the frequencies are beyond the set filter and they decrease in volume accordingly.

What is roll-off? And why would I want a gap? Doesn't that mean I am going to be missing out on some of the music? Also, should I use my head unit's LPF also, or just turn it off?

LPF @ 80Hz
HPF @ 125Hz
let roll-off handle the gap. it's less than an octave gap, and roll-off will make for a smooth transition without causing a warm spot.
no overlap. you want just the opposite.

If it were me, I'd turn both LPF's to 80hz and the HPF to the same.