I have to ask a lot of questions in lieu of detailed info.
Does the M-Box have -10 / +4 pad switch? if you have it at -10 switch and see if this helps
OK- the gain on the pre-amp typically dictates the recorded level, while the fader controls the "output" level (to master, monitors, headphones). You set the level of the gain on the mic "Ideally" so the signal does not clip and preferably is generally around -18dB to -10dB.... many people try and record as hot as possible, however this creates problem when you mix and especially makes mastering a nightmare. Recording "hot" reduces/eliminates headroom which means you have little to no room to work with for EQ, compression, etc. Also, with condenser mics, the more gain you use, the more room noise you're likely to get, so "louder" may result in more unwanted noises. Pushing the PT fader past 0 can also introduce clipping (distortion) as the signal to the master out is a summed signal of all tracks, and even though you may not have any individual tracks clipping the master output could be, which again results in distortion
"I" would adjust the pre-amp gain/level to the above mentioned levels, and start the 'fader" at about -10db, adjusting as needed but never above 0 . IF you're singing style is to scream a dynamic mic may be a better choice over a condenser; IF you have strong plosives (excessive air movement fro 'p" sounds, among others) you will definitely benefit from a pop screen/filter.
Once this is set up, note the levels of the input. The wave form should correspond to this.
bounce to disk and burn to a CD and see if it's louder on pother systems or still quiet.
What are you monitoring through?
Mono is if you want the output to be mono and not in stereo, has nothing to do with the mic input