need some to build a sealed sub box.

With the given specs for that sub, it looks like doing a sealed box would be a fine choice to make.  There is no benefit in either of the extreme sizes given;  those, remember, are the limitations in size.  Build your box so that it fits somewhere between the extremes, as indicated by the 1.25 cf size given in the manual.  I would likely tend to construct a little larger, such as 1.5 cf, knowing that it is easier to reduce airspace in a built enclosure than it is to add airspace.  Reduction of airspace is simply a matter of adding displacement materials, and that can be accomplished after setup and listening tests sometime in the future (if it is felt to be necessary for such experimentation).

There is no balance between SQ and SPL.  It's either one or the other, and that decision starts with what you buy and how you listen.  SPL is traditionally considered a competitive term but is generally used to describe a system that one might consider "bass heavy".  SQ is generally the goal for someone who appreciates the whole range of music; doesn't put the subs on a pedestal above all else.  Your descriptions point to SQ as the goal and you should realize that and not feel as if you have to give a nod to SPL.  We EXPECT the system to be LOUD...there is a volume knob to turn it down when you have to.  SQ is enjoyed very much loud.

As for you comment on the front amp's gain, I'll say it like this:  set the gain where it is supposed to be set.  Balance between front soundstage and sub is controlled otherwise through the deck.  Use the sticky thread on setting gain and search for prior threads regarding this subject to make sure you do it right.

Excellent advice all around. I'm learning and every bit of advice is fully appreciated. I agree that Pioneer likely recommends 1.25 cubic feet as that's a decent middle ground between the extremes. That's what I'm going to go with as a base box size... and as you suggested I may add just a little up to maybe 1.35 cubic feet to allow for extra polyfill/bracing in the box.

On that note... Pioneer also says in their manual to line the enclosure walls with sound absorbing material. Do I go to the extreme here of using something like Dynamat or equivalent type product on the enclosure walls... or does just stuffing polyfill material into the sub achieve the same effect? I was also wondering if the box is filled with polyfill... is there no potential problems with heat dissapation if a fairly large amount of the polyfill is surrounding the sub assembly inside the box? Maybe these are noob type questions but better to know the best way to proceed now while I'm learning so I can learn and do it right the first time.

My concerns (and comments) about the gains on the amp used to power the front comps are because I realize I have sizeable amp (200 RMS x 2 channels @ 4 ohms) that's capable (if used stupidly) of potentially blowing the 6.5" comp speakers which are rated at 85 RMS (260 watts max power... but I disregard any "max power" ratings anywhere on amps/headers/speakers etc as I realize they are fantasy land figures.) I realize the volume knob is the best way to control the potential to destroy speakers.

Line the walls with something like egg-crate foam or sound absorption material.  Dynamat is not what they're talking about (although it works well inside enclosure for other reasons.)  Using poly-fill will also help and no, there is no concern that it reduces cooling.  As to your gain question, please read the How to set your Gain post in the Hot Topics forum.