I will just ignore everything that has been posted so far and rephrase my answer from the XMPlay forum:
OpenMPT has removed the "normalize" function for some unknown reason. However, it will be there again in the next version. You could use the old MPT 1.16, which still has normalize.
XMPlay will play all mods as loud as possible, but that's not our intention with a tracker.
Also, you can normalize your wav files in an external application like Audacity.
I also want to rephrase that the Unreal soundtrack only uses the S91 command, which is normally encoded as Pro-Logic surround in most trackers and mod players. I guess that's even the case in the Unreal engine. You'd normally need a Pro-Logic decoder to get this kind of surround, if you don't, you will just hear a surround-like effect because the waveform of the left side of the channel is flipped by 180 degrees on the right side. This is normally how surround is emulated on a stereo setup. I have not checked it, but I guess MPT doesn't use the rear speakers when you use S91, because you would have to use the S9B command to pan in the rear. So the quad output should be the same as stereo output.
All in all: I don't even think that the Unreal soundtrack was meant to be played with four speakers. S91 was (and still is) just a common effect to create a sound that's "somewhere in space", because it will confuse the listener's ear. Even in stereo.