For many diehards, Led Zeppelin were even more mind-blowing onstage than they were in the studio. With 1997's 'BBC Sessions,' that argument was strengthened considerably, as it showcased the most powerful quartet in rock music at their most playful and hard-hitting.

The now 15-year old collection (which also included a third disc of interviews in its limited initial run) is split into two distinct but equally excellent halves. The first disc comprises a wide range of material from the band's first two studio albums, along with a handful of unreleased cover tunes, recorded at various BBC sessions throughout 1969. The second disc features most (but, controversially, not all) of a blistering 1971 concert at London's Paris Theatre.

The performances on disc one are unanimously essential. It's fascinating to hear Zeppelin toy with the arrangements in a proper studio setting, adding subtle new shades. 'What Is and What Should Never Be,' for example, finds Robert Plant singing through a wall of reverb, with fuller vocal harmonies. The surging, punk-ish 'Communication Breakdown' is even more distorted and explosive than its album counterpart. But the true highlights are the covers, particularly a groove-heavy revamp of Robert Johnson's blues classic 'Traveling Riverside Blues,' built on Jimmy Page's searing slide-guitar.

The live versions on disc two aren't always perfectly played, but they were always passionate. 'Going to California' is filled with awkward transitions and fumbled lyrics, but its mandolin-fueled balladry is incredibly poignant. 'Heartbreaker' is even grittier and more propulsive than the standout version on 'Led Zeppelin II,' particularly with John Paul Jones' amped-up bass.

'BBC Sessions' still faces some criticism for leaving off tracks (like the harmonica-boosted blues of 'Sunshine Woman'), in addition to the edited performance of the lengthy 'Whole Lotta Love' suite. In 2009, Page remarked to BBC Radio, "For those who were hearing it for the first time, there was a certain guitarist who was really wincing at the out-of-tune part, because the string had dropped and it was out of tune ... I was gyrating on the chair here wishing I could tune it up. Anyway it got tuned and it got back in, but I could hear then it was putting my playing off."

But even with its controversial editing, 'BBC Sessions' should be celebrated for what's there -- the greatest rock band ever in all their imperfect perfection.

Listen to Led Zeppelin's 'Traveling Riverside Blues' from 'BBC Sessions'

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