Crossover Tuning - Amp settings

Ok tuning gurus, here's a quick one for everyone:

Setup:
Front - MB Quart 2-way component with crossover unit (RSC216)
Back - JBL 6x9 3-way (GTO935)

Questions:
1. Since the front components already have a (passive) x-over unit, should I bother putting amp that powers these speakers into High-Pass mode?  or should I just leave it on "Full"?  How is this set in most (good ) installs?

I've tried it on both settings and it does seem like I can turn it up louder (without distortion) when i set the amp to High Pass, but it also sounds less "full", less warm.   Not sure if it's just a "cleaner" sound, or if by setting the amp to High Pass (at approx. 70Hz), I'm introducing a dip somewhere in my frequency response.  (also, not 100% if the it's actually distorting on FULL mode or if it's just that those mid frequencies below 70Hz make it sound a little messy)

2.  What about the rear 6x9 speakers?  I'm a bit confused about what the rear x-over setting should be.  Some argue that the rear is for "rear-fill" purposes only and that high frequencies from this location ruin imaging, so this means using a High Pass setting would be incorrect? (obvious question?)  I've had it on 'Full' for awhile and recently set it to High Pass (again, at approx 70Hz).  Not sure if this is the right setting, but the bass sounds like it's a bit "cleaner" now.  Could it be that the bass from the 6x9's was interferring with the sub in my trunk?  (single 12" Orion XTR2 DVC sealed box)  If so, then what setting other than High Pass would be correct for rear speakers? 

Anyhow, I know this is a long description, but any help and personal experience would be much appreciated. 

It all depends on your tastes and your equipment's capabilities, but as a very general answer, I would suggest your should use the high-pass filters on your amps.  This will keep the low bass the speakers can't reproduce very well anyway out of the components and allow the amps to not have to work so hard trying to play them.  Your amp will run cooler and cleaner and be able to play "louder" without distortion.  Is the filter a fixed setting or variable?

As far as soundstage is concerned, this is again a matter of taste, but for a traditional SQ setup, the soundstage must be in front of the listener.  The rear speakers, if they are used at all, are for "fill" and effects.  If both the front and rear sets are driven by equal amplifier power, I suggest setting your fader control to favor the fronts by about 25%.

70 Hz is a bass frequency, not a mid, by the way.  So if your bass sounds cleaner with the filters on, then it is possible your sub is out of phase either electrically or physically with the main cabin speakers and by eliminating the low bass from the cabin speakers, the subs were able to be heard.

Cheers!

DYohn,

The amp setting (Hifonics Centurion and Hifonics Taurus) is variable.  There is an analog knob to change the range from 150Hz - 70Hz.  I understand that 70Hz is a bass frequency, but even at that, the sound change is pretty drastic at this setting, as if there could be more (lower sound) going to the speaker--as if there is very little sound coming from the 6.5 mid portion of the component.

It's difficult to tell whether my sub is out of phase because I've tried wiring it both ways and I can't really hear much of a difference.  I think in most cars, you're going to get some cancellation no matter what because none of the speakers are directly (perfectly) facing the same direction or perfectly facing the opposite direction,  (ie the speakers on the doors are facing in, while the speaker in the rear dash is firing up, while the sub in my trunk is firing to the rear - if you map out the sound waves from each of the speaker sources, several sources cross orthogonally). 

Yes, it will be noticable in the Quarts, as they can go down to about 40 Hz (although not very efficiently.)  I always cross my front mains at least at 100Hz, as any driver smaller than 8" really does have trouble with frequencies lower than that.  It can create a hole in the sound, though, if your sub is crossed at 40Hz or something.  The calibrations on most amps are approximate crossover settings, by the way, but close enough.  Try this:

Set your JBL GTO's to "Full" and disconnect the line level input to the amp driving the Quarts.  Play a track that you know well and has good bass program material.  Now switch the high pas filter on for the JBLs and go listen from your driver's seat.  If the bass gets more clear or louder, your sub is out of phase.   If the bass gets softer or less defined, then the JBLs are adding significant bass information and should be left in "full pass."

If you hear a problem in the above test, try switching the phase on the sub and see what happens. 

I'd setup the Quarts with a high pass at 75 or 80 Hz if it was me, but then again that's my taste!  And by the way, if the output from the Quarts seems low, perhaps you have them out of phase with one another, as they are great sounding speakers, especially in the mids.