Recommended sub settings

The muddiness is probably not the result of an improper crossover setting. It's more likely caused by room resonances that just weren't being energized before you added the subwoofer.

Make sure you're not cranking the sub too high - at first it feels very gratifying to hear all that bass, but if it's too hot your mixes will end up being bass-light. Measurement is the only way to take subjectivity out of the equation.

As for what the crossover should be set to, that depends on your main monitors. 30Hz is almost certainly too low.

You want to have the sub kick in just above the lowest reliable frequency your main speakers can deliver. Look up the specs in your owner's manual. Hopefully, the manufacturer has provided an actual graph so you can see where the -3db point is. If not, you can determine it by measurement. What you want to do is set the crossover frequency just a little above that frequency. Since the knob isn't calibrated (I have the same sub), it's a bit of a guess so err on the side of too high so as not to leave a hole in your frequency response.

Far more important is where the sub is situated in the room. Moving it by a foot or two can make all the difference. Keep in mind that it's not necessary to position it exactly midway between your main speakers - it's OK to be off to one side. If it's anywhere near a corner, get it out of there. Try to position it as far from any wall as possible (difficult in a small room, I know).

Once you've narrowed down the candidates for where to put the sub (usually dictated by the room layout and all the other stuff you have in there) start experimenting with different locations.

The sub is big and heavy and a hassle to move around, so rather than move it, move your ears instead. Plop the sub onto your mixing chair and start crawling around on the floor with music playing. When your ears find a good spot, where the bass sounds less boomy and more defined, that's where you'll want to put the speaker.

Like Bitflipper said, positioning is everything. You could try this ...

Set a tall stall in your listening position. Place the sub on it at about ear hight.  Play a sweep oscillator through  the system (say 30-180 Hz) and crawl around the room on you hands and knees till you feel you are hearing the subs as best you can. Put the sub there. It's very rough but it worked for me.
CC

Sorry bitflipper.. I am a bit of a novice when it comes to to this stuff, so I am not exactly sure when you say "see where the -3db point is".  How is something like this found?

I am a bit of a newbie when it comes to the more complicated stuff that I have not dived into fully yet.

I understand, but how does that measure the sub to be a -3db? 

I have a sub... Polk Audio 100w 10" ... it really adds to the quality... I have it set to "take over" at about 100hz and it's actually down pretty low.

The muddiness I think is likely coming from the lower midrange frequencies as opposed to the lows, especially if you currently have the sub set to 30hz.... that is mighty close to the lower end of human hearing.

Having a number of midrange instruments, piano, guitars, in the same range playing can easily muddy up a mix. also, going flat across the mids can let too many mids into the mix. I almost always pull my mids and lower mids down...(scoop the mix) and this one technique really helps keep the mix clean as opposed to muddy.

NOTE: I mix more by ear than looking at db levels...although, looking at the db levels does tell you alot about what is going on.

I increased the frequency to 100hz and it seemed to make a difference, now I think I had the sub too high in my room because when I went to my car and listened, the bass was missing..all mid range and highs were coming through.  I guess I need to turn the sub down a little lower in volume then when I mix?

Damn, I really hate this forum software. My response just vanished, including links and references and a really clever joke about Ghadaffi.

M-Audio claims a frequency response down to 56Hz for the BX5a. Frankly, that sounds a bit optimistic for 5" drivers,

I had a look at the CX5 manual and it says: "50Hz-30kHz (+/-3dB); -10dB at 50Hz"  I'm not sure how 50 Hz can be both "+/-3dB" and "-10dB" at the same time. Perhaps it wasn't meant to be a factual statement.

So maybe the BX5a speakers are similar. They have a frequency response from "56Hz - 22kHz" (according to the manual) but it might just be at -10 dB at 56 Hz.

If manufacturers can't be bothered to publish meaningful specs, why should we be bothered to buy them?  And yet... I have 4 pairs of BX5, and a couple of BX5a kicking around here... I think they sound ok, but their caps go bad quick. I've had to replace ones on every set. Doesn't speak well to their manufacturing/QC.