How can you make a rock vocal sound better?
Admit it... you were expecting a magic answer to this one. You were going to get advice on what plug-ins and effects can turn an ordinary vocal into that of a true rock god. Or goddess.
The key fact about rock vocals is that they are an established musical form. If someone invents a better way of doing vocals for rock... well they will be some other style of vocal, and not true rock.
So you have to look at the defining parameters of the style. Just as you wouldn't record hip hop without an MPC60 or the like, and you wouldn't record country music without a Fender Telecaster guitar, then you shouldn't record rock vocals without a Shure SM58 microphone.
And that's all you need. You don't need a fancy preamp - just plug it into your mixing console, set the levels and record.
Now if you really want to get better rock vocals, what you need is a great rock singer. You could of course fire your current singer and get a better one. Might as well fix the drummer while you're at it.
Or you could get your singer some voice training. Rock vocals demand great control. They might sound out of control but there is a hell of a lot of precision work going on in that there larynx. The art is to sound loud and aggressive while letting the mic do most of the work, and developing the muscles and tissues of the throat over a considerable period of time. Even the great rock singers took time to develop. (And many over-developed too quickly leading to surgery to correct serious problems).
One technique that is used all the time in rock is the 'fry'. This is where the soft tissues surrounding the throat and larynx are made to vibrate coarsely and roughly. I'm sure you know the sound. I'm sure you know also that you don't have to do this for long before you get a terrible sore throat. Look forward to an infection the next day and possibly a trip to the doctor for antibiotics.
Building up the technique to produce the fry on the regular basis required by rock demands competent instruction. You can do it alone like the ancient greats of rock, but it will be a hit and miss affair - and no-one knows about the rock singers who didn't make it because they shot their voices to pieces before they were ready.
You know, great singing will sound great with any decent microphone and any competent preamplifier. We hear so much about equipment that we forget that the source of so much music is in fact human.