I've been experimenting to fake a Mesa Mark IIC+ with the American '90. I found i nice sound with SEG and BIAS to the max so i started reading about BIAS on valve amps, especially Mesa ones...
It tourned out that an higher BIAS warm the sound and a lower BIAS make it raw, so:
1) high BIAS for warm soloing leads
2) low BIAS for cutting edge solos and metal rythm
As a final result i got a pretty decent fake of the IIC+ sound that i used to make a patch that sound like Rob Balducci in the clinic i've recently been.
Here is the preset:
Next i will be sperimenting to obtain a flat clean sound and to obtain further more result with my bias settings...