You would use the line level out of your soundcard to get signal to a poweramp / speaker setup.
If your preamp "reacts" as well as you say it does and sounds as good as you say it does you could even make a hardware version for live use. (never thought of that did you?)
Most amp sims allow you to turn off spkr sim in case you want to use a real poweramp/spkr setup. (you wouldnt need spkr sim cause you would be using a real spkr)



