[Ultralight]

Apologies if:
1. This has been discussed in this circle. I did a search and did not find an answer to my question. 
2. If this violates some any purist sensibilities regarding full range needing to be absolutely full range without modification.   

Question - if I have full range speaker, but I want to EITHER high pass or low pass the speaker, what is the way to do this that allows the 'purest' signal to get through so that I loose least quality such as resolution, dynamics etc.

Again, apologies to any purists here to may really disdain this question....no offense intended.  I'm a relatively newbie to full range have been smitten by the first full ranger I've tested extensively.

Thanks,
UL

[FullRangeMan]

To low pass use a inductor, best are the ribbons, Jeff will serve you well:
http://www.soniccraft.com/index.php/inductors-12-gauge-c-27_94
Calc the inductor here:
http://www.erseaudio.com/CrossoverCalculators

[shooter]

What driver are you using? What is you intended crossover point? Do you intend to bi-amp or tri-amp? I have a couple of friends that have special output transformers wound for their amps to roll off the low frequencies to match their drivers. Or If you have a capacitor coupled amp you can calculated the cap value between 2 stages for the desire roll off. On my set up I run my full range drivers wide open and use speaker level active low pass filter for the subwoofer and add a high quality bullet super tweeter just to enhance the top end.

[JLM]

To be a purist:

1.)  Don't (there is no perfect speaker, just find the compromises that work best for you)

2.)  Crossover at the lowest/highest possible frequencies to achieve whatever your goals/needs are

3.)  Use a 1st order (6 dB/octave) filter to avoid phase mixups or infinite order (brick wall) filter to avoid phase hearing the same frequency from two sources

Extended range drivers make for some of the best midrange drivers.

[Duke]

1.  Figure out where the goal posts are, and start out by making choices that will allow your to reach them. 

2.  Take into account everything you possibly can. This includes driver thermal and mechanical limitations, off-axis as well as on-axis response, magnitude and phase of the impedance curve, the speaker's acoustic environment, amplifier characteristics, and everything you know about acoustics and psychoacoustics. 

3.  If possible make your own measurements and learn what they are telling you, and what their limitations are.

4.  Once you really get into it, the drivers will tell you where they want to be crossed over, and with what slopes.  Don't be locked into textbook approaches - for instance, if your drivers tell you they might like asymmetrical slopes, try it.  Remember that in the real world, the net rolloff is driver rolloff + electrical rolloff. 

5.  Expect to go through many iterations on your way to getting it right.  Even with a professional-quality measurement system and computer modeling program and years of experience, I have never gotten a crossover right on the first try. 

6.  In my opinion, don't worry too much about being true to the purist ideal.  Effectiveness is the measure of truth.  For instance you may have planned to cross over at 100 Hz, but by golly it sounds better crossed over at 500 Hz, well then do that.