heres what I learned.I'm gonna start lower than you asked,and add on the features.
5435 I'm including this one because the display is so much better than the 8 volt ones. The crossover offers 80,100,and pass(off).Slopes are preset at 12.This is a "global" crossover,meaning it is a low pass for the sub,high pass for the front/rear.1 choice only and it does both high and low pass.
8445 It has the same "global" 80/100/pass,but slopes can be chosen.0,6,12,18,24 are offered.I could tell no difference between the "ADVANCED" mode and the "NORMAL" mode.
8455 PRO mode off offers the same 80/100/pass and slopes as the 8445.
PRO mode ON: You choose speaker mode.If you choose 4speakers+sub(Non fade): Front 63,80,100,125, and on up.you can also choose the same 0-24db slope.Then Rear same as front.Then W(woofer) 63 up to 200,0-24db slope.
If you choose 3 way speaker mode:front is tweeter any freq,0-24 slope,rear is mid woofer you can choose high pass freq,and slope,low pass freq and slope.
W is sub.you can choose the same 63-200 and 0-24 slpe.
CAUTION if you are in 3 way PRO mode and turn the PRO mode off,your tweeter will get full range and fry. You are supposed to set the volume to 0 before you do this.
You may ask why would I do this?Certain settings,like fade,balance,non fade volume,CSII(whatever that is) and other stuff are not available on PRO.Likewise,bass,mid,and treble are not availible with PRO off.
I'm gonna rant now.They took the rubber off the NOBs and they are slick.The display is small and dismal.This really bothers me becuase I'm old and don't see well.The display for Sirius has lines of info that requires a magnifying glass.The RCA's are not gold plated.It has the stupid video stuff in the display,and too much crap on it that is buried in the sound control,and a lot of stuff is not available in 1 mode or the other.I find it not very user friendly.
The Sirius sounds great,the head unit sounds great,and offers the cross over flexibility that I needed.I wanted to use small,nigh quality amps under my seats,and those don't have crossover control.
I have an ECD-416 from 1997 that sounds al least as good as this 8455,but it has no crossovers and no AUX input.It sold for $800 8 years ago,so I think things are going downhill.I can't fit my Adcom amps in my Element,and it breaks my heart.They are the best amps I've ever heard.I'm gonna seek out some old high quality amps that will fit and try them.Maybe ADS,PPI,Proton,Nakamichi.
If you need more specific crossover freq info,I can look it up.At this point I'm trying to get the non-fade volume to work as it seems to have no effect.The instruction book is like a novel.
I'm not 100 percent sure on the 8445,but we could get no other crossover stuff in it and could find nothing else in the owners manual.
Hope this helps,dan