Yeah, that's what I meant. Apparently I type faster than I think. :)

My onboard audio devices are also disabled, so I have no clue.

I had the same problem with my 18i20 in respect of other audio programs not playing back when Sonar was open. I solved the issue by right clicking on the speaker icon and choosing recording devices and deselected the 'Allow applications to take exclusive control of this device', same with the playback.
If that doesn't work let us know and I will see if I changed any other settings.

biodiode
I had the same problem with my 18i20 in respect of other audio programs not playing back when Sonar was open. I solved the issue by right clicking on the speaker icon and choosing recording devices and deselected the 'Allow applications to take exclusive control of this device', same with the playback.
If that doesn't work let us know and I will see if I changed any other settings.

Nope, that doesn't work either. I can have iTunes/YouTube open at the same time and both will play back audio simultaneously, but if SONAR is open nothing else will play audio unless I close SONAR.

I'm not sure how much credence to give to reports of Firewire incompatibility when it comes to the latest generation of adapters, specifically pci express card adapters. I would think these are new enough that early generation 1394 chips aren't used in them anymore. But I could be wrong.
 
 
I'm using an SIIG 1394 (VIA chip) PCIe adaptor from newegg on my i7-6700/Z-170/Win10 to a MOTU Ultralite.
I can play SONAR and Sound Forge simultaneously, both using the MOTU's ASIO, while you-tube grinds away using MOTU's WDM desktop default interface. That's two ASIO plus a WDM all at once, steady as a rock, no clicks, pops, or any of that business.
 
It's lazy driver writers that fail to implement a multi-client capable driver. There no reason to be limited a single ASIO client, unless you're trying to pull a fast-one like Presonus, using driver time to process pseudo-monitoring-plugins as in their AudioBox VSL stuff. Complain - you're entitled to.
 
Cakewalk's 'Share Driver's with Other Programs' option is simply a work around for those stuck with single-client ASIO drivers.

Just an update: 
 
Received the Focusrite-recommended FireWire card with TI chipset in the mail, plugged it in, hooked up the Saffire, and bingo. Works flawlessly.
 
So now I've got my loopback back, the ability to run multiple audio programs simultaneously, etc. Although that particular card had been known to have some compatibility issues with some Z170 boards, evidently my board isn't one of them. :)
 
Happy camper here!

Good news! You are also on Win10 now? I forget the complete course of events, but the potential loss of my discontinued Saffire makes all system alterations subject to intense scrutiny.

Yep, Windows 10 here.
 
Long and short of it:
 
Built a brand new PC (specs in signature) with a clean install of Windows 10 Pro (64-bit). My particular Z170 board (ASUS Z170-AR with newest available BIOS & chipset drivers installed before OS installation) works great with the new FireWire card recommended by Focusrite. (http://www.amazon.com/DP-Firewire-800-PCIe-NN-FW0012-S1/dp/B001EQCNMW?ie=UTF8&psc=1&redirect=true&ref_=oh_aui_detailpage_o02_s00). I didn't install any drivers for the card, just plugged it in and Windows found it. Went into Device Manager and was happy to see "Texas Instruments" listed as a IEEE 1394 device. 
 
Installed the newest version of Saffire Mix Control then plugged in the 40 Pro...worked like a charm.
 
Only caveat - had to buy an adapter so I could plug in my 400 cable into the 800 ports on the new card. :)