orcish75 wrote:

Jesolo is the resident expert on Aztech cards, he has a bunch of them, so hopefully he'll see this thread and comment on it.

I fully agree with orcish75's comments.
Of all the Sound Blaster clones out there (from that time period), I believe that the Aztech Labs range of sound cards came the closest in matching up to the Creative Labs sound cards.
User James-F also did some research and analysis on these cards: Aztech Sound Galaxy cards
Earlier models of the Aztech Sound Galaxy range (from the 1st generation cards) did have the low pass filter (like your NX Pro, Basic 16 & NX Pro 16 range of cards). They also had built in Disney Sound Source/Covox Speech Thing support (albeit, only in mono). However, they were unfortunately a bit more noisier than your later generation sound cards.
Personally, if I had to choose one of these Aztech cards for my retro machine, I would go for one with the AZT-2316 chipset (despite not having a low pass filter).
Stay clear from one with the AZT-1605 chipset, since these do not fully support Sound Blaster Pro stereo output for voices & sound effects (they do have an OPL3 FM chip that outputs stereo).

despite not having a low pass filter

Not a big fan of low pass filter on Sound Blasters.

It's a standard feature to deal with aliasing.