Hmmm...the combination of the 58 and the MP aren't particularly noisey, but they aren't particularly clean, either. And compression, by its nature, will bring up the noise floor of the track you are compressing.
If you are using the same mic and preamp for all of your tracks, your also getting a build up of the same noise. Nothing to do about it but record your tracks as hot as you can while minimizing noise at every stage of the signal chain.
(Forgive me if you already know some or all of what follows: )
Lets set this thing up. This is how I check my system for noise.
Plug the mic in and start singing.
Start turning up the input knob and watch the leds. On the MP the gain button only needs to be used if you have to turn your input knob past, say, 2 o'clock to get a decent input level. If you have the input turned up that far and the "Cln" led still isn't giving you much action, turn the input knob back down, hit the +20dB Gain button and start turning it up again.
Turn it up until the "Cln" led at least stays on- or moves a little into the "warm" section if you like. Don't let it light the "Clp" led even during the loudest parts of your song.
Ditto on not needing the Phantom button- that's for condensor mics. Your 58 is a dynamic mic. Leave it off. (button out)
Then hit the phase switch a few times. Usually one way sounds slightly better than the other- I'm not sure why.
Then check in with your recording program and see what level is coming into your Gina. Turn the output knob of the MP up until your input meter almost but doesn't quite hit 0dB during your loudest parts. If you have to turn the output knob on the MP much past 3 o'clock that may be where your noise is coming from.
Record a test track. Listen carefully for noise, especially in the spaces between words. Once you know what the noise sounds like, leave your mic on the stand where it is leave all your setting the same, close all your doors, tell everyone in the house to be queit, turn off your monitors and put on the headphones.
Turn them up LOUD. Leave the mic on as if you were going to be singing into it but don't touch it or go near it. The feedback from the headphones will blow your head off. Listen for that noise. It just might turn out to be your computer humming or the sound of the fridge vibrating through the floorboards. If so, throw all your blankets over the computer and unplug the fridge.
Next try turning the input knob down a little and the ouput knob up a little. If that doesn't change the noise, try turning the input up and the output down. Find out where that noise is coming from. Fiddle with the knobs just a little to see if you can get it to go down a bit. Don't move them much or your levels for recording will be too far off. Also, if your soundcard has an input stage do the same test.
OK. Turn the headphones back down to a reasonable level and record another test track and see if it made any difference. If your levels are different for the two tracks, be sure to use your mixer to make them the same level for fair comparison.
Then, the most important steps of all:
Take the blankets off your computer before it melts and plug your fridge back in before your milk goes bad.
Other ideas for reducing noice bleeding in through the mic:
Your 58 does a great job of ignoring whatever is behind it. Make sure it is pointing exactly away from the noisiest thing in the room.
Try moving the mic around the room to see if you can get a cleaner sound.
Good luck,
Chris