There are quite some mistakes floating around on how to judge a saturators sonic quality and here are some tips to avoid the most common pitfalls:
1. A good saturator does not appear as distortion in the very first place. Firstly it just saturates incoming audio signals which means that at a similar RMS output level it simply reduces the peak performance (which results in a smaller “crest factor”).
2. This immediately implies that you need a RMS meter in your output chain to compare different saturation settings or devices to another. Basically this is the same when comparing limiters or maximizers.
3. Distortion is a side-effect which typically occurs at higher saturation levels. It can have different sonic qualities, e. g. due to the frequency distribution of distortion which makes a huge difference to human hearing and if the effect is perceived as to be rather gentle or not.
4. Don’t rely here on a simple spectrum analyzer since it does not know nothing about the concept of being “gentle” or not.
Summary: Always assure equal RMS output levels and then use your ears.
For some free RMS tools look e. g. for Smart Electronix DFX RMS Buddy or Voxengo SPAN which also has a RMS and crest factor indicator.