OK... here's what I was testing:
The two attached images are screenshots of both versions of the "SBP-MIX.EXE" utility. Version 1.00 has the stereo/mono feature, while version 2.00 does not.
The attached MP3s:
There are two music clips for each test... #1 is a longer four-track version, while #2 is a shorter version with only a single percussion track playing.
Each clip has been recorded 2 times... the first with "Mono" enabled, and the second with "Stereo" enabled. I've ALSO downmixed each of the recordings to mono for comparison... it makes it a bit easier to distinguish the subtle effect of the stereo.
Anyways...
Compare "Tetra_Compositor_Song_Clip_1_SBP-MIX_Mono_Mode_Original.mp3" to "Tetra_Compositor_Song_Clip_1_SBP-MIX_Mono_Mode_Downmixed_to_Mono.mp3". They sound identical.
Secondly, compare "Tetra_Compositor_Song_Clip_1_SBP-MIX_Stereo_Mode_Original.mp3" to "Tetra_Compositor_Song_Clip_1_SBP-MIX_Stereo_Mode_Downmixed_to_Mono.mp3". You can hear the difference of the stereo effect if you listen closely.
Thirdly, compare "Tetra_Compositor_Song_Clip_2_SBP-MIX_Mono_Mode_Original.mp3" to "Tetra_Compositor_Song_Clip_2_SBP-MIX_Mono_Mode_Downmixed_to_Mono.mp3". Again, they sound identical.
Lastly, compare "Tetra_Compositor_Song_Clip_2_SBP-MIX_Stereo_Mode_Original.mp3" to "Tetra_Compositor_Song_Clip_2_SBP-MIX_Stereo_Mode_Downmixed_to_Mono.mp3". Again, you can hear the difference of the stereo effect if you listen closely.
Oh... of course, the reason that the mono MP3s have a higher frequency response is simply because the Sound Blaster Pro plays stereo mode at 22KHz, while mono mode plays back at 44KHz.
Note that I ALSO tested several other programs (a few games, as well as Fast Tracker II) with the "SBP-MIX.EXE" utility in stereo and mono mode. NOPE... there was NO stereo effect whatsoever! This stereo effect only seems to be heard in Tetra Compositor.
BTW, I loaded the resulting recordings into GoldWave, and it appears to me that the stereo effect is simply shifting the left channel a wee little bit to the right in time (maybe 2 milliseconds or so), resulting in the stereo effect. As Cloudschatze said, sort of a "pseudo-stereo" effect.
Conclusion:
Who the hell knows? Maybe Creative Labs intended for you to run the "SBP-MIX.EXE" utility and enable stereo, and then run Tetra Compositor... although this appears to be undocumented (?).
Nevertheless, I've always thought throughout the decades that the stereo effect was SUPPOSED to be like on an original Amiga running "ProTracker" or "Ultimate SoundTracker" (e.g. L-R-R-L panning for each of the four tracks), but apparently not (?).
P.S. Attached is "Sound_Blaster_Pro_Mixer_Control_v1.00.zip" for anyone who might want it 😀