Sounds like you are plugging the mic directly to your recorder, I am not familiar with it but many mic preamps on all in one recorders are not really great quality.
The SM58 is a dynamic that takes a bit of gain to push the signal through the recorder, so you probabaly have to turn the gain up on thepreamp pretty high to get a listenable signal. if that is the case, it would be no surprise if there was a lot of hiss in your recording. THe one thing about teh sm58 is that it isn;t very sensistve more than a few inches away, so it probabaly would be less likely to sound terrible from the room. This is probabaly a gain staging issue.
Gain staging has to do with making sure all the equipment (microphone, preamp, recorder, any other items in your chain like effects) is in the ideal range for that pice of equipment. If the microphone has a weak signal to pick up, it will sound like crap, if it is too strong the mic will distort. Most preamps have a range of gain where they sound best, some can even add great tone to your signal when in the range. The same goes for most of your equipment (but youahve a simple setup, so leave all that out).
In your case, make sure, as sillyhat says that the mic is getting a clear and loud signal (SM58 is hard to get too loud) and that you are close (0-6 inches probabaly is a good place to start) to the mic.
Next look at your recorder, and make sure there is not a selector for your inputs mic/line or +4/-10 or the like. If there is such a thing, try switching to see if you suddenly get a much stronger signal than you have had up till now, check the manual for details here.
Next look at the knobs in the input/ preamp section of your recorder- you normally have two or more ways to make this louder, a "gain" knob and a slider or knob for the tracks volume. The gain controls how much boost the signal gets befopre the recorder starts actually listening to it. The mic puts out too little to record, the preamp will boost this to something the recorder can work with.
Only then will the slider or knob for the track volume come into play. Back to the gain knb though- the trick here is to give enough gain to get the signal toa usable level without adding additoinal garbage to the signal.
The blurb you mentioned about most preamps saying "low noise" or "noiseless" is garbage. If every pre gave gain without adding noise, the expensive ones would be a lot less popular. The key is how much clean gain you are likely to get. After a certian point youa re adding dirt to your signal .
So you can see that if you don't feed the pre a decent signal (sing louder) you will have to add more gain and you will end up with too much dirt in your recording. the best way to gauge this is to put the fader for teh track at "0" usually about 3/4 of the way up. Then while providing a decently loud source (sign or speak into the mic close, and sufficently loud) turn the gain knob up until you can monitor (headphones or whatever) a nice strong signal.
If you are recording digitally, you want the signal you are recording to hit (input meters) about -18 or so on the meter. This will probably be much less loud than you would expect, turn up the monitors to compensate.
If you can't get it loud enough to record a fairly clean signal, you have crappy pres on your recorder and need an external preamp to get the signal loud enough to record without your recorder's budget pres adding dirt to your signal. The presonus tubepre might be an optoin, but there are probabaly better ones for the same or not much more money.
If i totally misread you and the signal is clear, but you are picking up stuff in the actual room like a computer fan, or the dog snoring or something, then you might actually be putting too much gain on the mic and it is picking up extra noise as a result. Try turning it down a bit move your recording spot, aim the mic differently, and treat the room.
Good luck,
daav
