More than likely the hiss is coming from the microphone but you can reduce that dramatically by working with your various stages of volume and recording levels.

Microphones all produce what's called "self-noise" and there's not way around that. However, that doesn't mean that you can't control it to some degree. Start by reducing the Channel Gain on your Mobile Pre. Then reduce the volume on the channel you're recording to in your software. Also open your control panel in Windows (or OSx) to see if you have a control for the volume on your USB port. If so, reduce that as well. You should be able to come to useable sound levels.

The "right" level is the hardest part of the game, I think...

It does sound like something may be turned up too much, or down too much - somewhere..?

Goal is to:

Get the highest possible level from mic to preamp(Using the preamps input gain controls), without distorting. This will give you the best signal to noise ratio, which should mean any hiss, hum, far-in-the-background noise should be diminished.

Next, set the levels for every other piece of gear in the sound chain, from the preamps output level to the computer itself(Or other "target" device - tape recorder?), in the order of their occurance. With every following piece of gear/device getting the highest possible level without distortion.

This is tricky! On the first 3 "setup" run-through's the singer sings at "A" level and all is well and good. On the first "take" he sings at A++ level and now there is distortion(No "hiss" though!)... It's tricky. Many use compressors and limiters for safety, but this includes its' own set of hazards.

To playback, do the reverse. Highest possible level without distortion, adjusting from source to speaker - of course you can turn down the speaker amp if its' too loud then.......

All this assumes the equipment is the "right" equipment, for instance that everything is +4 or -10, etc. and that the gear is hooked up properly and operating correctly.

It's a process... A l-o-n-g process......

Many gear manuals give good examples of initial level setting(Mackie's mixer manuals, for instance - available free, on-line for your educational study.), in a variety of "systems" use... When all else fails, might try it "their way".....?

TG

Does the pre-amp draw power from the usb?

What you are hearing is probably room noise.

Try recording something in a closet or under a large, heavy blanket draped over a couple of music stands. Did the "hiss" go away or lower at all?

If it did, it's time to treat the room.

Check out the acoustic forum if that is the case.