Hm..Here are a few things that may help:
1. You could look if there's background noise in the recordings and try to remove it. That will make the voice clearer in any case.
2. trying different resamplers and using the one that makes the voice the least muffled. I wouldn't recommend bkh01.
3. Equalizing the voice after rendering
Here are some professional equalizing tips: "Boost the volume of the upper-mid or lower-treble ranges (between 3000Hz and 5000Hz), depending on the singer.
Cut the volume at about the bass ranges (150Hz) No VOCALOID can sing that low, nor would it sound good to boost it if they could.
If your unvoiced consonants (like F’s and S’s) are hard to hear, boost the volume around the treble (10,000Hz and up)."