Henley,
The site russ gave you is a good start and will be a good reference for net and mag based reviews. For auditioning systems here is a scheme I personally use. One audition method I use is the
"John Marc0 Distortion and Coloration Determination Method" for short JMDCDM
For distortion, all music is possible bec of distortion (that's why we can talk bec of distortion our voice box makes). I usually group them into two kinds, good distortion and bad distortion.
Before I listen to one particular system, I make it a point to audition it with music material that I have heard a hundred if not a thousand times. That material now becomes my baseline, and I can consider that as my subjective "neutral" reference. Regardless of what distortion the material have, it becomes neutral as it becomes zero reference point.
So when I audition to a system, I mentally listen to tone cues and points, this where distortion becomes really discernable, then I make a judgement if the distortion is annoying? if yes then it is "distorted" and usually the most common form is clipping. Where the tone just won't reach it's peak or a peak is boken, dispersed or plain "basag or wasak".
If it is not annoying then I will conclude that it is a form of coloration. Coloration can come in forms, it can be veiled where it seems like a blanket of tones are prevalent throughout the material. It can be "matining" where high frequencies supercedes other lower frequencies and will be overwhelmingly dominant in the material.
It can be harmonic, subsequent tones of musical instruments (i.e. tone decay from a pluck of a string instrument) somehow heavily masks other parts of the material i.e. vocals.
Remember this is done comparative to your mental baseline reference.
Now, how will I judge systems? sometimes I will say it is heavily colorated, where distortion is not annoying but deviates from my reference. Or it can be transparent where it is close to my reference point thus it's almost neutral. Or it can be plain distorted, where the whole thing is annoying to my ears.
As you go by, you create your own method and come up with your judgement points. But always remember, your reference point maybe someone else's bad distortion and vice versa. Be sensitive in how you convey your opinion, you can be offensive if your not careful. Don't be trapped in self righteousness, people will stop auditioning with you.
Another approach is diplomacy where you can use flowery terms and it can be a good conflict avoidance tool. No harm in using 500 dollar words for a 2 cent opinion.
Good Luck