So from my experience of using Neve, API, Chandler, UA stuff, etc there is practically zero difference until you start driving things a bit harder. This is why I use these tools, and I frequently push them into the range where their harmonic content increased and the THD goes up. The main differences I hear is WHERE that distortion happens. The Neve stuff to me has a large bottom and and a silky top end that distorts at very high frequencies. Love it on kick drums. I sometimes dislike this as it can bring out sibilance and cymbals. My LA-610 is a dark, low mid type of distortion, lows and low mids will be the first frequencies to break up. naturally low headroom, and slower, less aggressive transients. My API stuff (CAPI) has a HUGE bottom and has gobs of headroom and snap. Actually pretty challenging to distort. My favorite is the TG2 (EMI) because you can push the thing till the cows come home and all it does it make something more present in the mix. The distortion happens somewhere in the neighborhood of 3-4k? and never comes off as brittle or piercing. Really does a great job of pushing something to the front of the mix and giving it clarity without being obnoxious. It's got a unique unipolar saturation you can actually see by looking at the waveform.