I've been off on a Vai mode for the past three weeks, *and have found some absolutely killer sounds out of my Marshall DSL 50 watt 2000 head. *I'm also using a Ibanez TS-9, *Boss DS-1, *and an Ensoniq DP/4 for delay/reverb.
There are a few noteworthy things about what I have been doing:
1) I play in a home studio, *so it is not out of control with volume
2) The signal goes TS-9 --> DS-1 --> Amp--> effects loop (DP) --> amp
3) I have found that maxing the gain on the clean channel, *while keeping the volume low on the amp is Vai like, *especially when using the TS-9.
4) TS-9 settings: *Tone 30%, *Drive 80%, *Level 75%
5) For true high gain sound (still on the clean channel), *I keep the tS-9 on, *and also click in the DS-1.
6) DS-1 settings: *Tone 10%, *Level 100%, *Drive 60%
7) For a more saturated dist sound, *I turn off the DS-1, *and switch to the Marshall lead channel.
I've had great results by using the Lead 1 setting on the Marshall with Gain at 90%, *and once again volume low.
9) The signal BEFORE all of this is going thru a Bad Horsie Wah.
As I said before, *this setup is quiet if need be, *or bodacious tone in your face by just turning up the volume on either channel. *It is an extremely dynamic rig, *which is*required to play a lot of Vai sounds.
I also switched cabs from my Marshall 4x12 to a Mesa 2x12 w/Vintage 30s - that was a very sweet change.
If I were you, *I'd try the TS-9 and DS-1 in front of what you are doing. *I hate playing my amp without the TS-9 option, *and recently am very psyched with the DS-1 as well.