hi
There is no such a thing Plug In pre amp. It's not possible. All the so called Pre Amps in UA are post processing
after the converters. Basically what they do is adding some EQ, compression and distortion trying to simulate the sound of pre amp. This can be achieved with any plugins (EQ, Comp, Distortion, Saturation)
They call it Pre Amps but it's not how pre amp work
Real pre amp is an Amplification and/or Impedance change. If you connect a Mic into line level input, no matter
what pre amp plug in you put, it never gonna make it work.
What you need is to use an Analog Pre Amp of some kind. It can be build in your sound interface or separate one.
hope this helps