Can you post an example?

Very few people have the sheer volume to over-power an SM58. And the rest of your gear all seems to be very decent as well - KRK and Presonus should all be up to the task. Unless one of those 3 things is malfunctioning, you should be OK.

I'm wondering if this is an issue with recording technique or singing technique.

Anyway, a sound clip would be very helpful. verse & chorus.

Finished Thread.

Mic technique should vary when singing very softly you might be two or three inches from the mic , very loudly 4-6" would not be unusual.
The adjustable input volume is referred to as gain and should be checked in your DAW as to how much of it you need. In Cubase LE (I'm not sure which version you are using) you activate record and monitor in the channel strip and there is a narrow cyan band that will show you what level you are tracking at. You want enough so that you can get a good recording in the quieter parts but still have headroom (below clipping) in the loudest parts. As to what you are referring to when you say Cubase is turning something down I don't have a clue what it is you are talking about.

Just stumbled upon this thread...and forum for that matter.

I've never used the firestudio but am betting that it has it's own control panel seperate from Cubase, or any sequencing software for that matter. It should show you the levels of the inputs. You must first make sure you aren't clipping at the interface level.

Using compression at the input stage is a somewhat controversial subject but many people choose to add compression pre-interface. This can help eliminate the peaks and clipping.

It may also be a good idea to record your verses and choruses (or softer parts and louder parts) on different tracks and adjust the input gains at the interface. You can always adjust the volumes when you mix and automation is a must.

And, the number that you say "turns down" when your track gets louder is actually the level of the track. This is a decibel measurement, 0 being clipping. Generally the software wont put a negative sign before the number but it is implied. Generally the range here is -infinity to +6. You never, ever, ever...ever, want this number to reach 0. 0 is clipping. Generally, and this is going to seem a little too quiet, but you want to record you vocals with the peaks between -15 and -10. This will give you enough headroom to work with good compression, EQ, reverb...and anything else you want. All effects alter the gain structure of a track so you really don't want to record as hot as possible. I hope this helps.

Finished Thread.

Right on. And as dvdhawk said, it would be really helpful to hear a sample of the issue. Record a verse and a chorus exactly as you have been. Let us hear what you hear and believe to be the most problematic.

As for compression...the compression in Cubase is a software plugin, and is great. However, being a plugin, it does NOT alter the actual wav structure. It takes the sample and runs it through a series of algorithms before it hits the output. In your case however, an external hardware compressor would be the way to go. You would put it in the chain BETWEEN the mic and the interface, adjust it to where you want it (which is an art-form in itself), and then record. The recording here would not be dry, it would be compressed and unlike the software plugin, there is no undoing this compression. I love compression.

That being said, you shouldn't NEED compression pre-interface. Honestly I'm shocked that you are having this issue with a 58...they are impossible to overload and it sounds like you tried turning things WAY WAY down. I have recorded with 58's with complete success (although to really achieve good vocal tone you are going to need a large diaphragm condenser, and not a cheap one).

If you are not able to get at least a good recording with the SM58 then you probably need to review your recording technique especially gain settings on the interface and/or your mic technique. While a great condenser will likely give you better tone it will not solve the volume issues you are experiencing and won't make up for a lack of knowledge or experience in the recording process itself.

As far as using the compressor included in Cubase it would do you a world of good to read the manual that is available in PDF format in LE by clicking on help icon. There is a section on VST plugins and the compressor specifically. You are going to continue to have issues if you don't understand how to use your program.
There is a Firestudio screen you can use to check your input levels with greater detail than the small inset available in the channel strip of your track or you can open the mixer in Cubase which will also show where your levels are tracking but be aware that these levels are set only at the interface itself not in the computer.