damping is the second most important thing you can learn when playing slide, absolutely essential, but there is a lot of subtle tricks too. once you have damping down cold, then you can play with selectively undamping notes or passages. if you watch derek trucks very closely youd be surprised how much he doesnt damp behind the slide.
the most important thing to learn is intonation.
on big stages when you can crank the 50w amp and not blow the rest of the band off the stage then high wattage amps work fine but not many of us can really do that. i dont think its overdrive or distortion but the compression that you need. if you had a great old ross compresser then i think you could play on a spanky clean amp and make it work fine. smallish tube amps tend to compress nicely which lends itself to being a great amp for slide. old funky cheap off brand tube amps can be fantastic slide amps for some things. old tv front fender amps are some of my favorite slide amps, they mush out just right. i used to use a '56 pro for some slide stuff and it was heaven.