The XLR outs are at mic level and the 1/4" outs are at line level, so that would explain the difference in levels you're hearing. The XLR outs are really designed to be plugged into a channel strip on a mixer with a mic preamp. There's nothing wrong with using the 1/4" outs. They're unbalanced, so using them with a long cable run could add noise, but you should be fine plugging into your studio speakers with them.