No, I cannot hear any high pitch sound when I enable the Lowpass filter in the mixer tsr.
The filter definitely does work when I use the Sound Galaxy's own diagnostics (SG2.EXE) and playback the "SB VOICE TESTING". There is no impact when I playback the Disney Sound Source or Covox Voice Testing.
What driver version are you using? I'm using version 2.41 from the Aztech FTP site (the one for the SG NXPRO Extra), but I haven't noticed any difference if I use version 2.11 (the one for the "standard" SG NXPRO).
I also have a version 2.3 that someone posted a while back, and this one's menu's (on the configuration - SGPFIG.EXE) has a different colour background (similar to my Basic 16's)
If you have the original CPS Sound Blaster Pro 4.0 disks, then perhaps I can test my card with those (my card looks very similar to this one - http://www.amoretro.de/2012/11/aztech-sound-g … tml&prev=search.)
You could perhaps also try using a different IRQ (IRQ5) and see whether that doesn't perhaps solve your "high pitch" problem?. My NXPRO flatly refuses to use IRQ7, since it's conflicting with my parallel port. Only if I change the IRQ jumper on my interface card, will it use IRQ7. Strange enough, I don't have this problem with my Basic 16.
I think I need to get the proper drivers for this model number (being the CPS driver disks).
This card is not exactly the same as the original Aztech Sound Galaxy NX Pro - the Aztech model does not have a 16-bit extension, whereas this one does (at least, according to some pictures I've seen).
Also, my card has a jumper for an EEPROM setting, but done of the driver disks I have writes an entry into Config.sys for the EEPROM (like my Basic 16 does).
Don't quite follow your last post - How do you determine if the stereo is reversed or not, if the stereo reversal always happens when the card initializes?
I must admit, when I test the sound card's voice playback with "SB VOICE TESTING" using the Sound Galaxy's own diagnostic utility, then the stereo is not reversed (regardless whether the tsr is loaded or not).
I'm still a bit worried about my Basic 16's filtered output - might have to test in another PC before I perhaps consider a recap.