Yes a sub will make it better, you want to go for one with a built in electronic (adjustable) crossover and amplifier.
BUT the trick is that you need a preamp with preamp out / power amp in
So it depends on your current preamp and power amp or integrated amp...which do you have? Does it have pre out / power amp in? This determines what sub will work on your system...
If you do not have pre out/ power amp in, then you can go for a passive sub that works from the existing amplifier...but does not have adjustable crossover.