Hawkson, i believe in what alot of JohnK has already mentioned.
Here's my adiditional 3 cents.
Four speaker "stereo" is a bit of a flawed idea.
Stereo is 2 channel, period (i won't argue this point overly hard but there are true stereophiles out there who would consider your words true blasphemy).
The idea of taking a single forward imaged sound (2 speakers up front firing in one direction) and then playing 2 opposing speakers in the rear is not only confusing to your ear since the original 'stereo' sound was not designed for 4 speakers in such an orientation, but the sound waves themselves will have cancellations potentially giving you this stagnant sound that you perceive. Think of it as two waves of water colliding. Molecules of gases that makeup air in motion (sound waves) very much have an effect on each other and not always in a harmonious way.
The idea about using 'surround' sound with 4.x or 5.x channels, is to try and create the most realistic way of hearing an image as our ears would normally perceive sounds all around our head. If a band were playing in front of you, it should not sound as loud coming from behind your head as it does in front.
Using a full channel 4 speaker surround creates this non-realistic sound.
If you are trying to create a well imaged sound from any source, i suggest you try using the Dolby Pro Logic modes first which utilize real recorded information for the surround speakers, left and right back, and see what this does to the sound you seek.
(NOTE: For the nitpickers out there, I did not say DPL has distinctly recorded channel info for the surrounds)
Secondly, try listening to some music off a dvd soundtrack that is using DTS or Dolby Digital 5.1 to get an idea of how the surround channels should really sound in comparison to the 2 main front speakers.
Using the all channel surround mode is ok, but i personally have never found it enhances music or tv watching whatsoever. If anything, i find it distracting.
The only way i ever got to try and enjoy the full 4 speakers in surround (and yes, they were 4 direct radiating speakers, 2 M60s and 2 M22s), was to sit in the VERY middle of the room equidistant from all 4 speakers on all angles (e.g. the middle of the square).
Most listening rooms are not setup with a couch completely in the middle, with equidistance from all speakers.
"Those who preach the myths of audio are ignorant of truth. Research begets reality."