Most likely there is nothing wrong with your machine. It just sounds like maybe your signal is just already pretty hot coming in. You might need to describe your setup a little more to get some input on what might be going on. I assume you are plugged directly into the phono inputs? Or are you going from a mixer output into the s2400? Does your turntable already have a phono preamp built in? What gain multiplier are you using?
Also, the reason for being able to hear 1/2 on 3/4 is given in the post below.