For the cleanest signal path, put it directly to your soundcard. You would only put the preamp through the mixer if you wanted to mix that signal with others before recording to the computer. In cases where you do use the mixer, make sure you turn off the mixer's preamps
I generally put mine through the mixer just because its easier to control and i dont feel it changes the sound much if you dont put gain on the preamps. Of course it depends on what kind of mixer you have?
What kind of preamp did you get? I found the TRS direct out on the SP (awesome i dont remember the model ) doesnt work too well so ive ran it from the XLR out into the mixer.
The phantom power has nothing to do with turning your preamps on or off. It has to do with turning your phantom power on or off.
Basically youll wonna keep the gain level at Unity. This is usually marked by a U or a 0. Some mixers actually allow you to bypass the preamp but i think thats only in more expensive models but you can check that out. With the gain at U or 0 you wont be giving any more gain to the signal which allows the the least possible re-preamping possible although you will still get a little of the preamp coloring. This coloring like i said should be minimum but it will still be their, which is why it is just better to run it directly to input. But in my case its nothing i worry about too much.