I have a small Peavey tube head with a USB output that the manual says you can record directly out of ....I plugged into an open USB port on my pc, but SONAR doesn't seem to show it as an audio input source....My audio interface is a Forte and it is the only IN/OUT option when arming tracks...is there something I need to do in the SONAR side?

The recognition of USB devices is a function of Windows itself.  Once Windows loads the drivers and recognises the device, only then Sonar will be able to recognise it.

What I would do is go to the website of the manufacturer of your Peavey tube head see of they have a driver for your version of Windows.  Your manual should show you the web address.  Download and install that, then you will see it listed in Sonar.

promidi
The recognition of USB devices is a function of Windows itself.  Once Windows loads the drivers and recognises the device, only then Sonar will be able to recognise it.

What I would do is go to the website of the manufacturer of your Peavey tube head see of they have a driver for your version of Windows.  Your manual should show you the web address.  Download and install that, then you will see it listed in Sonar.

Thanks! I will do that..

Also don't know if you're using ASIO, but start off with MME. It's the "lowest common denominator" for audio interfaces, and then you can work up from there.

You can't actually use multiple audio interfaces at the same time if you're using ASIO (the most common pro driver standard on Windows). Selecting it as your input in Sonar preferences would mean turning off your Forte. You would have no sound output.

+1 to the above, you would need to use a driver mode other than ASIO for SONAR to see both devices simultaneously.
 
However, that USB output is most likely for people without an audio interface... As you have a Forte, it would be preferable to route the amp through it to take advantage of the Forte's ASIO drivers (least latency). Be sure to use the proper output from the amp for this though (check the manual) and bring up levels slowly on both the amp and Forte to verify results first.