Just got a Scarlett 18i8 and successfully installed it. Unfortunately, Sonar won't let me set the bit depth to 24. At least, when I set the bit depth to 24 it resets to 16 when I reopen Sonar, telling me that "The default audio format is not compatible with one or more sound cards." The Scarlett is the only sound card I have enabled (the motherboard's built-in RealTek is disabled). When I run the wave profiler it tells me that 24 bit is an option for the Scarlett.
 
So, is Sonar defaulting the maximum available to the disabled on-board sound card, or is something else wrong? Maybe I haven't successfully plugged the Scarlett into a USB2 port on my computer (they're not labelled, but usually I get a message when I've plugged a UBS2 peripheral into a USB1.1 port).
 
Any ideas are appreciated - thanks!

SONAR will use what is availble, so start with Windows sound settings (with the Scarlett plugged in and online), verify it is default and "exclusive mode" is not set on the properties page.

Next open Focusrite MixControl software (be sure to download/update drivers, firmware and software from their site), and set the values there (these should match what you set in Windows).

Lastly, launch SONAR and check what is available to you in the audio preferences. You should see the interface there, but may need to reassign it as default if more than one are listed.

Thanks - I reset the Scarlett to not be in "exclusive mode." All drivers are newly downloaded from the website. I don't see in Mix Control where bit depth is set, and there's nothing in the documentation about it. When I load Sonar, all of the RealTek inputs and outputs are disabled, so all that's left is the Scarlett inputs and outputs.  Can't see where to assign a default interface, unless that's the dropdowns for Playback and Record Timing Master.

There's 2 places to check for 24bit:
Preferences-File-Audio Data-File Bit Depths - Set Record Bit Depth to 24
Preferences-Audio-Driver Settings-Audio Driver Bit Depth - Not sure you can change this, but it should follow the first setting.

I am on my cell here, so hard to describe well. Is Windows showing 24bit for the Scarlett?

In SONAR, the audio preferences should show what connections are active. Try shifting to WDM driver mode (allows multiple interfaces), disable all Realtek, etc. And only enable the Focusrite drivers. Then try shifting back to ASIO driver mode.

Ahh, under File it does show 24 bit for recording and rendering. it was on the Audio Driver Settings that I couldn't change it. Maybe I'm good - should probably do a test recording and check the .wav file to see. I'm currently using WDM because my last interface (el cheapo Behringer) didn't play nice with ASIO. Maybe I'll give ASIO another shot.
 
Thanks for all the help, guys!

Give ASIO a shot. The performance is somewhat better than WDM, and Focusrite knows how to write ASIO drivers.

http://us.focusrite.com/answerbase/is-my-interface-in-16-bit-or-24-bit-on-windows

Hey, switching to ASIO also got my Scarlett plug-ins working. Thanks, Bill!