Perceived volume depends on many factors. If you're just starting out, it may take a while to get your core strength built up to play 'louder'. Some players sound louder because they're able to dig in more due to years of practice. The strings do make a difference, however, most of it is down to your skill / technique / experience on the instrument. If you want to go as loud as possible (acoustically), you'll need high tension steel strings and the biggest bass possible, as well as years and years of solid practice. I've used Rotosounds RS4000 before, as described by Clarky, and concur that these aren't particularly 'loud', compared to other strings such as steels, but again, it depends how you play them.