then just use the analog outs of the VxPocket.
go to windows control panel> sounds, set playback device to VxPocket
open the wav file in your recording software (or 24bit capable playback software)
you may have to go into options to set playback device to VxPocket
open the VxPocket "mixer" (I double click the blue Vx icon in the windows "notification area", where the clock is)
adjust wave output level
the VxPocket has 2 types of analog outs, balanced XLR line level and 1/8"stereo mini headphone.
I'm guessing you don't have balanced inputs on your preamp, so you'll either have to use XLR>RC cables or adaptors, or just use the headphone out with a 1/8">RCA cable.
You can adjust the headphone output in the VxPocket mixer.