The best thing to do is get a pair from a dealer to test out in your setup. I've heard some people say they sound "punchy" but other people really like them. It's true that there isn't specific equipment for R&B and rap music but you need sufficiently low frequency response from your monitors to be able to do it correctly. The Mackies are +/-1.5db to 39Hz which is actually really good, but if you're doing really bassy stuff you're going to need to pick up a sub. And make sure you test your studio to make sure you're not boosting bass frequencies due to room accoustics or speaker placement. You can get a sound pretty level meter for pretty cheap which would let you setup things correctly.
I'd suggest taking a CD with similar music to what you'll be working with to a dealer, try out a bunch of monitors, then take a few sets home for a weekend to see how they sound in your environment. A lot of monitor selection is what you think sounds good and what you think you'll be able to work with.
