Unless you are looking at far field monitors (main monitors), I would just look for something that goes down to about 55 hz. Whats really important here is two other things. First, it doesn't matter what the specs on the monitor say, only how they sound. All the specs on the different ones may look pretty similar, however, the sound is radically different. Many speakers that say they go below 55 (if they have an 8" or smaller LF driver) really don't. Technically, they might, but whether or not those frequencies are usable in real life is a completely different story. Much of this low frequency response is actually dependent upon your room acoustics. On the other side of the coin, there are speakers out there that sound like they exceed what their chart shows. I used to have a pair of
Dynaudio BM6's. They were only rated down to about 55 hz. It always amazed me how if I went around the corner, those little 6" drivers really sounded and felt like there was a sub running. However, those ultra low frequencies (that felt good) really wasn't usable when sitting anywhere near the sweet spot. A big part of how the monitors sound is going to be based on your budget. Monitor's can be really cheap, or really expensive. I went for a mid priced sol;ution in my studio. Certainly not TOP of the line, but definately not cheap. I went with passive
Dynaudio BM15's. At $1100 a pair I find them to be a real bargain. When you factor in a mid range amplifier, they run nearly the same as a set of Mackie 824's. However, in my opinion, they shouldn't even be lumped into the same class as the Mackie's. Of course maybe the next guy that comes along doesn't like them. Its all opinion.
The one thing I can say though, if you are serious about what you do, if you want to continue doing it, get a decent set of monitors ASAP. You will never regret it, and it's the one tool that really affects everything that you do.