Well, I use an Audigy and the ProGamer edition, and tried out the grenade test you did keo in UT 2004, but in Battlefield 2 instead, using OpenAL.dll (ver 6.14.357.11). There are no specific 5.1 settings in the menu though, at least to my knowledge, but I got exactly the same result as you... Great sound to the left and right, but weak and tinny especially from the back. I can accept the sound being weaker from behind, "lower in output" as Matt wrote, but with less bass? No, and especially not from the front as is the case.
Wouldn't the best thing be if someone could record a session, for instance with a grenade test front/left/right/back, post the files here and also send it to Speed-Link/Creative? Preferably with samples compared to another headset, such as the Zalmans, as some say they have proper bass all around, or from people with Medusas that don't behave like this.
This would accomplish several things:
Firstly, Speed-Link could confirm to us if that's the way it's supposed to sound, the way they want the design to work or if something is wrong with the sound picture.
Secondly, everyone on the forum could understand what the problem is, and as it seems that some say they don't experience the lack of bass, they could listen to the samples.
Thirdly, and best of all, we can all can agree on what we're discussing and that would give us a common platform to continue the disussion from: design or incorrect behaviour?
I mean, when you talk about image quality you usually post a picture to show graphical anomalies; the corresponding thing with sound anomalies should be sound samples, shouldn't it? I could do it - if I had the recording equipment, but I don't, so I pass on the task to someone brave enough to boldly go where no man has gone before...
/m
PS Also, I played Medal of Honor: Pacific Assault set to 5.1 and EAX 3, and just out of fun turned 360 with an engine humming nearby - the sound from the engine totally disappeared when it was at the back of me. I would have tried Q4 as well, but unfortunately, at the moment I don't have space enough to install it. But I wasn't bothered by this issue in Q4 as I am in BF2. Maybe it's application dependent?