You could try selecting mid-gain on the rear panel, and setting the lead gain pulled and set fairly high 6-8 to give good signal into the tone controls but turn the lead drive well down to 2-4. Also try leaving a reasonable amount of treble on the amp so that the upper mids and picking detail comes through, then fine tune top end with the tone controls on the guitar - I dont know but I suspect that he uses them quite a lot. Bass and mids I set vey low (3 &2) and lead presence pushed in and fairly low around 3. I would guess that his sound comes over more overdriven than it really is because of the long sustain from the guitar and his fingers and probably pretty high volume, and that the amp gain is much less than it sounds.
I haven't tried to get the Santana sound but I have spent a long time tweaking the iv to my idea of the early Boogie lead tone which is based around an early Mark iib that I used to hear (with drive pedals). I find that even moderately high gains on the iv tend to soften the lead sound and lose the detail and attack in the top end and upper mids which I think the earlier marks had and may be important in Santanas sounds. I actually prefer to set the lead channel like this to give a bright bluesy crunch, then use a boost for leads, rather than piling all the gain on the amp.
Just my thoughts - - -
