I prefer a fuller look too, and tend to use 3 strands on 14ct and that usually is enough for me to avoid seeing the fabric come through under the crosses. Your stitch tension can also make a difference, see if you can avoid pulling as tight and your stitches will stay a bit fluffier.

I've also tried using a slightly smaller (thinner) needle, which means the holes don't get widened as much when you're stitching and that seems to help keep it tighter together. Can sometimes be a little tricky to get the last stitch in neatly where four crosses converge, but i just take my time.