Most jam bands are long on vibe, short on tunes. Gov't Mule, led by singer-guitarist Warren Haynes, write solid rock songs in the power-blues, heroic-vocal tradition of Free and Led Zeppelin. For half of this unique two-CD set, the Mule go long and hard on their own, hitting a studio-album peak in the hard-times barrage "World Boss," the slow-burner "Whisper in Your Soul" and the fuzz-lick tangle of "No Reward." On the second disc, they revisit that material as heavy sidemen, backing a dynamic cast of guest singers including Grace Potter, Jim James and Deep Purple's Glenn Hughes. Ever wonder how Humble Pie would sound with Elvis Costello up front? Find out in "Funny Little Tragedy."