I don't try to nail his tone, but a Strat into the clean channel of an amp pushed to where it starts to break up and goosed with a Tube Screamer just happens to have a pretty strong resemblance.
It's true particularly with him, that a lot of the tone does come from the way he plays. His tone is pretty basic Strat tone. It's the way he bends and slides and other little nuances that set him apart.
So the BB Preamp and Blue Collar don't really come to mind? Do they sound a little overly processed. I have both and they are "ok", but I actually haven't found THE alder strat for me yet so it's hard to judge those pedals with respect to this thread.
I just went to a big blues jam a few weeks ago and plugged my Anderson Hollow Classic straight into my Mesa Lone Star Classic with slight grit on Ch 1 and Ch 2 with drive set pretty low for a throaty lead tone. Not really trying to emulate SRV tone at all, as that's not my forte, but the tone was just freakin' awesome for that vibe.
I gots many comments on the dynamics and sustain and chime. Mostly from dudes with real bf Supers and Vibro Kings asking what pedals I was using...
Well, you might want to look into it. His tone varied. In the beginning he was using BF Fenders w/TS-808/TS-9's. Later, he was using the Steel String Singer. For the later tones, you might go for a pedal that can give you a dumble type tone. Maybe a Ethos or a Jeeter Gain stage red. I wouldn't use these as the gain tho, I'd use it to get that mid range dumble vibe. Maybe a tube screamer or Landgraff OD for the gain.
A lot of his tone also came from using multiple amps. I really don't know a whole lot about it as I haven't spent a lot of time emulating his tone. After listening to a few of his concert recordings it was obvious that the full, rich subtleties came from mixing together multiple amps, one with a more squishy mid section and at least one more that was voiced to get the highend layer.
I think the reason there isn't a straight forward 'gear' answer is that if you get an average guy to plug a strat into a tube screamer in to a fender amp it'll probably sound nothing like SRV.
Hand it over to SRV, get him to twiddle the knobs (probably up) maybe mess with the guitar (put a set of 11s or so on, get the action highish) and suddenly there it is - the sound materialises.
...I know people say he'd sound like him with whatever gear and all that - they're probably right, but I think he'd be close to his famous tone with a pretty basic set up, just set the way he liked it.
I find that my TubeScreamer through a tube amp that is already breking up is too much and starts to get me into Warren Haynes like territory. I think the reason why is most tube amps when pushed to that sweet spot really doesnt have that much more volume to it so it just compresses and gets smooth. Stevie played loud amps very clean and relied on his style to do the heavy lifting for him his TubeScreamer did very little boosting from what it sounds like to me. A great example of this is on the Texas Flood reissue, the one with the interview and the live tracks. Check out the live track of Testify, after the intro you can hear him kick on the TubeScreamer and it just adds thickness and girth to an already killer tone. Anyway to answer your question I have found the closest I have come to his sound is get a tube amp breaking up slightly when you really dig in and use a good clean boost like the RC Booster for just that little more umph. If you must use a TubeScreamer use less gain and more level. Good Luck all.