Reamping through analog input in SONAR

Sonar -> audio interface output -> reamp box (radial, palmer...) -> KPA input -> KPA output -> audio interface input -> Sonar

get one?
You´ll need one, an amp or the KPA wants to "see" instrument level at its input, not line level coming from your interface

only with the digital connections!

if you can afford wasting precious disc space with such a high sampling rate, but without hearing a significant difference to 44.1, you can afford this:
http://www.thomann.de/de/palmer_reamping_boxdaccapo.htm

I think the Return port can handle the line level signal, so plug your soundcard to that and use a mono loop before the AMP stack.
It's just ok.