In terms of speaker design, that is a fairly complicated question. Various slopes come into play because you are controlling how fast you want a particular driver to roll off and blend with it's complimentary driver. Phase is a little more complicated, and depends on the combination of drivers, but yes the x-over can be used to adjust the phase of the individual drivers. This may need to be done depending on the type and order of x-over used (Butterworth, Linkwitz-Riley, etc...)
Phase can always be varied with passive (in the design) or electronic x-overs, makes no difference price or type for the most part.
In terms of blending a sub, you generally want to roll off the sub faster than the main speakers because bass is more easily localized, and it will allow the mains to blend a bit more cleanly with the sub. Phase for the sub allows you to compensate for the sub's location within the room, relative to the main speakers. If the sub is near the mains, phase will generally be 0.
Your receiver will handle the x-over point and slopes.
If you are REALLY curious, there are a lot of good books out there. You might try
The Loudspeaker Design Cookbook for starters, and there is also the DIY forum on here.
BTW, you're not allowed to list your gear in your sig on this forum.
