How many times have I heard "fix it in the mix?" No amount of mixing skill will turn a sow's ear into a silk purse, despite what you read. Fixing it in the mix or in post is the most time consuming and least successful way of producing a good product. Getting it right during recording (or as right as possible) is the first step to good sound. If you learned in the old studio system the first real experience you got w/ recording was setting up mics, not mixing a paying client's masterpiece on a ssl board that you didn't know which button to push. Which is why I suggested learning the basics of recording a sound before learning how to fix a badly recording sound.
If the sound is booming, judicious use of EQ can help, but not cure, the boom. And since the OP has another open thread about "too much air" in the vocal, I suppose the fault lies within the recording technique themselves, not the stars, dear Brutus. If the same artist, same room and same mic produce such varying results, I'd start w/ the actual recording process. It is a learning process, so start at the source. Get that decent, and it is soooo much easier to get a decent mix. Get great sound, and the song mixes itself. If you can't change the artist, or room or mic, find the technique that produces consistent results. Then you can apply the same techniques to make the mix better. Otherwise, you are just spitting into the wind and hoping something sticks. Again and again.
Some practical mix suggestions for vocals in general, and your specific problem are:
Start w/ a high pass filter (which, as the name implies, passes the higher frequencies while cutting the lower ones). For female vocals, you might go up to 200 Hz and higher without damaging the meat of the sound. Play w/ the slope, if available, which is the curve of the filter.
Muddy or booming usually apply to frequencies a bit higher, up to 400 Hz. A separate notch filter, or for wide mud, a gentler dip, might help. Reverb and other time-based effects will muck things up, so add those only after the track sound is good or you'll be fighting the vocals on two fronts. For subtractive EQ like this, even the most basic EQ should work. You ain't trying to add anything special, just take away.