Here is a small video clip in which I play the Southbound Again riff, this time the studio version (I put another video online with the completely different live version a few weeks ago).
I played it in standard tuning. A few years ago I read an interview where Mark said that it was open G tuning (which somewhat surprised me here). Anyway, it works fine in standard tuning I think.
The amp here is a small tweed Princeton clone, no effects.