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I currently have a 12" sub in a sealed enclosure built to the factory specs. The box's internal volume is about 31L. Now it peaks at about 50-55Hz and it sounds good with most music, but when I play a bit of R&B and so on, the lower notes are very soft. Can one change this with a different enclosure or is it just the characteristics of the sub??

a sealed box with cabin gain factored in, gives a flat response.
this is why I usually suggest ported boxes for people in cars.. they tend to want more than "flat" for bass. the human ear is less sensitive at lower frequencies, so a bit of gain at that range helps compensate.

yes, go to a ported box. the volume will increase by about 50% on box size, but with a 4" diameter port in a 2cu ft box, you'll get a lot more low end from the same sub.
at least +3dB gain
you'll want the box tuned around 34Hz, and use a subsonic filter tuned to about the same frequency to protect the sub in the new enclosure.
do that and you're set.