Oh if using TB 3 PCIe 1x Asus Cards, or the 2TB one by Asus, the motherboard has to have a TB header on it.
That said, using a motherboard with onboard TB, will not accept the cards if it does not have that TB header on it.
Just to say...Setup on the right PCIe slot can be a tricky sortings..and dependency on what else is being used...
Let's say you use M.2...Might be when selecting this to enable in Bios, you defeat the PCIe 2 slot for , or other for TB.
Thus a conflicting state is invoked.
So you have to read the manual for the MB, Bios settings, and TB manuals....And it all gets confused in mind...contact the tech help by phone..ASUS ...And get it all sorted with a good techie, if so confusion is with the TB 1,2 or 3 setup on their MBs.
If it already has TB onboard...just a plain firewire to TB adapter should work with that, but because of the protocols being switched up by the adapter....it is not supported as stable with all devices & drivers .
Then again, you mention problems with signage of the audio driver, and MS.
So, there is a way to defeat , or address MS windows signage issues, if it cannot be gotten a proper driver for this setup you wish to use.
Not sure it can be solved ?..if the driver is being held under some form of policing of MS Policy agents services being interrupted or causing your problems with, by that MS service agent on that driver signage issue, is put, to protect the OS from harm , and your machine...But if that is the case....You will have to seek help from the Audio device manufacturing Techs for help, And MS Techies, to which I find have not solved my issues with this sort of setup, and finding signage issues with Motu.
If there Is a Firewire to usb adapter?...surely you can do the workaround until things are sorted?.
That would get you at least up and going, as usb and Windows are tied at the hip,...lol...Right?.
Well, hope that helps you also decide, and or look into all options of the lessor before spending on more so more...and finding down the road, they screw it up again once finding bliss.
Reminds me of Me...LOL.
Cheers.