Yeah, done all that. Been there, done that. Tried the card in every available slot, same issue. Changed BIOS settings around at Creative's request, same problem. Tried the card in my wife's computer, no problems whatsoever, works perfectly. Finally RMA'd my motherboard to ASUS. They had it for the past week and could find no issue at all. Ran full diagnostics on it and couldn't reproduce the problem. Of course, they wouldn't be able to reproduce it unless they had the same exact sound card, which I doubt they did, but I'm sure they did a decent job of going over the card. My Sound BlasterX AE-5 uses the same core, or maybe a slightly modified core as the Sound Blaster Z. When I really started researching the issue, I found tons of references to an issue with the Sound Baster Z based cards that sounded exactly like the issues I'm having. No sound after booting up, the card is often incorrectly recognized as a Sound Blaster Recon3D, requires multiple restarts to get sound back. In almost every instance, the users were told there was an issue, some sort of incompatibility, with their motherboard's UEFI implementation. If they changed from UEFI to Legacy BIOS, the problems went away and they never experienced them again. It makes sense now to me why the Creative Techs kept suggesting that I try booting into Legacy mode. They know there's been an issue for a while now, and there obviously still is with some motherboards. As recently as two weeks ago, another user on the Creative Labs forums asked for help with the exact same issue that I'm having and their motherboard is an Asus ROG STRIX Z270F.
Anyway, at this point, without actually trying anything yet, I'm all but convinced that this will take care of my issue.
What confuses me is why the sound card works perfectly on my wife's GIGABYTE G1 Gaming GA-H170-Gaming 3 based system. Her board is UEFI based as well, but the AE-5 works perfectly with her system. What also astounds me is that Creative has known about this problem, and that it's been present through at least two, possibly three generations of their sound cards, and they still haven't found a way to correct it. I just read today that Intel based boards will completely do away with Legacy BIOS and will be solely UEFI-based by 2020. What the heck are Creative owners going to do then? Everyone will have to restart their computers four times at boot to get their soundcards to work, just like I do now.
Anyway, I have my four restart fix down and I know it works, so I'm not in a huge rush, but I plan to redo my system within a week or two in Legacy mode and I'll post back what the results were.