When I wrote the post about DX8 fixing Redbook audio issues, I was also successfully using 2 sound cards in parallel:
-An ISA CMI8330 (all bug-free & flawless OPL3 clone, with good support for SBpro, SB16, WSS)
-A PCI Audigy2 ZS (best EAX experience possible for Win98 + very nice integrated MIDI for DOS)
For both of these I used their latest VXD drivers.

As this thread progressed, I did start wondering if maybe not all drivers/cards are equal.
And now @Duffman pretty much confirms this.