Well, the primary purpose of a preamp is simply to change the signal level - on a mic preamp, it converts the relatively weak output of a microphone into standard line level. Some preamps also impart their own sonic character into the sound, which is why they're used. In your case, you're just talking about MIDI controllers (the Maschine hardware doesn't produce sound any more than a MIDI controller - it all comes from the software on your computer), so you can't really connect them to a preamp - you'd have to route the output from your computer into the pre and back into the computer. Preamps are also generally pretty subtle creatures rather than giving a "distinct sound" - yes, they can make some things sound a bit nicer, but it's definitely not a one-stop solution to make everything better.