Plenty of good advice in this thread.
It seems you are fairly happy with the quality of the sound now, so before spending anymore money, why not try to perfect the set up you have now.
First, speaker placement:
I'd start by placing you centre speaker wherever is the most convenient. Then measure the distance between this speaker and your head where you normally sit while playing. (It helps to have another person to help with this.)
Then place your left and right speakers. You want to place them at the same distance as the center speaker, and the same distance again from each other. Basically form an equalateral triangle between your head and these two speakers.
Any studio people will tell you to then angle these speakers in towards your head to give you better stereo image, but personally I like to keep them firing straight ahead, as this will give a better phase correlation and give you a better near field sound.
Next place your rear speakers. Again the same distance from your head at about 10-15 degrees behind you. Point these in at your head.
All of this is great but is generally very difficult to fit properly around people's desks, rooms etc. The left and right speakers are the most important, so try to keep these setup following the equalateral triangle rule, and then compromise with the other speakers. (It may be possible to fix problems here software side. More later)
The other important thing when placing speakers is the Space Loading. Most smaller speakers are generally designed around a half space loading, meaning placed on a desk or attached to a wall.
Problems arrise when the speaker is placed on a desk right against a wall (quarter space), or worse on a desk in a corner of a room so a wall right behind it and right beside it (eight space), or the greatest crime, inside a bookshelf or such.
Because of the small displacement and low volumes of these speakers, only 15 cms or so is enough distance to avoid problems with space loading.
Again real life often makes correct placement difficult. The best thing to do is try to compromise and make sure all speakers are affected by the same loading. If all the speakers are affected in the same way then it won't affect the balance between the speakers, and it may be easier to EQ out the problems.
If you have walls behind the speakers it'll boost the low end, making the sound muddy. Walls running beside, or desks underneath extended infront of the speakers will boost the whole frequency spectrum but not evenly.
Once you have your speakers placed in the best possible way, it's time to look at ways to fix problems by affecting the audio signal.
Under 'Hardware and Sound' you should have Realtek HD Audio Manager or a similar program.
In this you will find two very useful tabs. Room Correction and Sound Effects
With the Room Correction tab, you can make adjustments for the differing distance between you speakers and your head. Setting the distance will delay the sound to make sure that the sound from each speaker will reach your ears at the same time. The volume adjustment allows you to adjust the volume of each speaker to compensate for distance and space loading.
You lose about 6db each time you double the distance. Adjusting for space loading is far more problematic, best just to play with it and see if you set a good balance between all speakers.
Under Sound Effects, you will find some reverb effects, don't bother with these, but at the bottom you should find an EQ section.
Unfortunately, there is no universal settings for EQ, so I can't give you any suggested settings. EQ is dependant on the reponse of the speakers, their placement in the room, the room itself and you own ears.
A good thing to do, is listen to some music through a set of good headphones, then listen to the same music again through the speakers, and adjust the EQ till you have something that resembles the sound in the headphones, or at least sounds good to you.
Please remember that speakers have limitations in producing certain frequencies especially at the extremes of our hearing range and trying to push these frequencies all the way up can likely damage your speakers.
Sorry for such a long read, but hopefully it helps someone.
On a side note, a couple people have mentioned using Optical s/pdif to run their systems. To my knowledge this only works in 5.1 when receiving Dolby Digital or DTS encoded sources found on movies etc, but doesn't work with computer games. With games s/pdif is only capable of stereo. Are the people using opitcal links actually running Elite in 5.1? If so how?
