this might be a little off topic but is a receiver/dvd home theatre question. What inputs should i use for better sound, coaxial or optical? also for video, s-video or component? right now im using component and optical and it seems to be best but i cant find anywhere which ones are supposedly "better".

little off topic...

coaxial vs. optical is a tossup. i use coax.

component is slightly superior to s-video.

It really depends on the frequency response of your mains in relation to your crossover point, as speakers with capabilities of playing lower frequencies are going to overlap you sub in the mid-bass area, and might cause some phase cancellation. I would reccomend finding out the actual frequency response of your mains using an SPL meter and an RTA rather then going by tech specs, which are usually off the mark, and adjusting your sub's crossover to have the slightest overlap possible.
YMMV. [/B][/QUOTE]

IMO finding and setting the optimum crossover frequency is a bit more involved than you suggest. For example, even if you have speakers that are capable of playing "low" there are advantages to crossing over at a higher frequency:

a) the demands on the power amps are reduced which can translate into greater dynamic headroom and cleaner sound;

b) the speakers don't have to work as hard either which depending on the specific speaker model can also yield some improvement.

Of course running the mains full and supplementing the bottom end w/ a sub can also work very well. It depends on the equipment, room, and user preferences.

If you are using the internal receiver/processor crossover you should definitely set the sub crossover to the highest possible setting; otherwise you may get some phase shift. Some sub’s allow you to bypass the internal crossover altogether; this is the best approach when using the receiver/processor’s crossover. If you choose to run the mains full, then you will need to use the sub's internal crossover.

As DVD_O_Rama suggests you should use a SPL meter and a disk w/ sweep tones (e.g. Stereophile test disc 2 or 3). Real Time Analyzer's are pretty expensive. But some stores will let you borrow or rent one. These tools can be very helpful in finding the optimum location for you mains as well as adjusting a sub-crossover.

Sure there is quite a bit more involved in getting an optimum setting between the frequency response of your mains and subwoofer's crossover point then what I posted, but due to the fact that a large percentage of the readers here are not hardcore tweakers and geekers, and most wouldn't know phase cancellation, room nulls and gentle slope crossovers from adam, I usually try and leave helpful opinions based on my personal experiences, yet not overtly technical.
Sure, it's obvious that higher crossovers will relieve your mains of having to push the extra low end, as well as saving your amps energy, I won't argue that. I have been running full-range speakers all around, with two powered subs for lfe in my current set-up for a while now, so it's been a while since I have had the displeasure of dealing with bass management issues, but the fact is, the crossovers in your processors are not brick wall, and there is going to be higher frequencies going over that. Most sub-sat systems use the 80hz crossover point in the receiver/processor which will still allow frequencies around 100hz to be sent to the sub. If you have the crossover on the sub all the way up, you're getting both the subwoofers and your mains playing frequencies around 100hz at the same time. I find this very unpleasant and distracting, as that high of frequency coming through a sub is easily generalized and destroys the intent of the soundtrack. This is one of the side-effects of using a processor's internal crossover, but one that most folks live with and to be honest, don't care to hear otherwise. But of course, YMMV.
Mine sure does.