Okay so i've had this problem forever (i use asio4all), every time i open certain programs like mixcraft (use it to use amplitube and play songs that need pitch shifting at the same time), amplitube (standalone), or guitar pro 6, i can't simultaneously get sound through each of them. Right now i want to play a song through guitar pro at a slower speed and use amplitube (standalone) at the same time, but with asio4all this isn't possible as there is massive lag and latency issues. Is there any way i can use guitar pro 6 and amplitube 3 at the same time through my Preamp?
Not with ASIO4ALL, part of the way it can be compatible with everything involves it running the device in exclusive mode (i.e. only 1 application can use it). If the preamp has a native ASIO driver (one made by the manufacturer) you might be able to use that.

You can slow songs down in most DAWs, I would be very surprised if Mixcraft lacked that feature. Otherwise you could slow it down in GP, export the .WAV, then import it into Mixcraft.