Hi Mike and Mike,
I certainly understand the principles that are illustrated in the diagrams above, but would it not also be possible to DI the guitar to one track in Sonar, dry, via the Hi-Z input of a sound card like the OP has (I use a QUAD-CAPTURE which is very similar) to one track, and then take a send from that track and route it out to one of the USB sound card outputs, through the to the cab, in the OPs case, or it could be an outboard effects unit. he output of the effects unit could then be routed back into Sonar onto another track.
This is different if you are using onboard effects, like GR5 or TH2, but for outboard effects, you need, at some point, to get the signal out of Sonar and back in again.
I'd be most interested in your thoughts.
In the meantime, I'm going to give this a go myself. I've got a Lexicon MPX500 gathering dust at the moment, and it cost me a tiny packet. It's got S/PDIF I/O too so I'm hoping I can keep everything in the digital domain.
cheers
andy