There are a ton of htings that could affect what you are seeing, and 99% of them are outside your mixer/mr8 package.
What kind of mic are you using? Read throgh the two stickied posts in the mic forum, and you will learn a great deal about mic selection and placement. Some mics are "bright" and will accentuate the upper ranges and be flat in the lows, you may have somehting like that. If you get a chance to try out some other mics you may find one that better suits your voice.
Our voices often sound different in our heads than outside as well.
Are you familiar with "proximity effect"? This says (basically) that for non-omnidirectional mics, once you are within a certian range, the closer you are to the source, the low end will be bolstered. depending on your mic, you should try to shift around the position of the mic and your head (with headphones) and try out different positions to see if you can get a better tone.
Move the mic and your head together around the room as well, if it is relatively small and untreated, the low notes reflecting around the room can create dead spots, and you could be standing on one. Some people reccomend this:
Put the mic and stand right in front of a closet full of clothes, witht he mic facing out into the room, and sing into it facing the clothes hanging in the closet. The clothes will absorb much of the noise going past the mic and avoid some reflections.
Hope some of this helps,
Daav