Robert Plant and Alison Krauss, Wembley Arena, London<br/>Sparks, Islington Academy, London

The figure on stage tilts his head backwards, thrusts his belt buckle forwards, shakes his hair loose and ... whispers gently. What's going on? What's happening is that Robert Plant, while Led Zep keep the world guessing whether the rumours of further live reunions will amount to more than merely hot air, is committing to this big-little tour to promote his little-big album with Alison Krauss, the bluegrass singer-fiddler from Illinois.

Raising Sand is a commendable addition to the corpus of what one might term "tumbleweed music". A collection of tastefully chosen covers from venerable American songwriters (Tom Waits, Gene Clark, Townes Van Zandt), it ambles along with the all-the-time-in-the-world pace of a tired packhorse: you keep expecting to hear the childish clip-clop of coconut shells in the background.

"Hammer of the Gods" this ain't. Feather duster of the gods, perhaps. Subtlety is the watchword here. The twangin', rock'n'roll single "Gone Gone Gone (Done Moved On)" is unrepresentative: apart from the heavy guitar clangs on "Nothin'", this is drowsy stuff, and Wembley's a long way to go for a snooze. The stage is insulated with nine large Persian rugs, of the kind you find hanging in the less-visited rooms of the V&A, lest the stamping of boot-heels wake anyone from their slumber.

They're a beauty-and-the-beast pairing: at just over half Plant's age, Krauss is, without being too crass, a looker, and most of Wembley has fallen for her by the finale. Stylistically they're more Kris and Rita than Kenny and Dolly, the aesthetic being Americana authenticity rather than Nashville gloss.

In the studio, Krauss has one of those soft, Seventies, serene Emmylou Harris/Linda Ronstadt voices, and for this project Plant too sings with an understated sweetness, harmonising at third intervals. Which is a pity, perhaps: the contrast between Krauss's coolness and Plant's coarse cock-rock roar would work well. Onstage, there's a little more dynamism, a little more June Carter perkiness to Krauss, whose screech on "Trampled Rose" threatens to shatter the glass on the Arena's safety doors.

The band, led by the legendary T-Bone Burnett (who gets his own solo spot for a couple of songs), is similarly rootsy. Grizzled musos, playing upright bass, pedal steel and big f-holed Fifties guitars, all give each other "this is good stuff" nods, the way people do when they're passing round a spliff.

Plant has, after all, always been a respecter of roots, in parallel with his life as a rock god: his Priory of Brion project, for example, and his appearances with Fairport Convention. Indeed, he made a point, at the recent Led Zeppelin reunion show, of confessing exactly which old blues records they'd plagiarised for each song.

Perhaps he's too much of a respecter, a fetishist of the form. Rhyming "money" with "honey" may have already felt trite back in 1955 when Dorothy LaBostrie and McKinley Millet wrote it ("Rich Woman", tonight's opening song). Half a century later it certainly does.

Plant may have told this month's Record Collector he has no interest in "warming my hands on my own legend", but he allows himself to bask in Zeppelin's afterglow once or twice: when the banjo player throws in the riff from "Black Dog", you initially assume it's a little musical gag, until it develops into a full-length rendition. It's quite wonderful.

A band who can be forgiven for warming themselves on the glow of their own legend are Sparks, the genius genre-hopping art-pop duo who have been criminally written out of rock history. In a typically inventive and challenging move, Ron and Russell Mael are performing the "Sparks Spectacular": a record-breaking run of 21 London shows in which they play every album from their career in order (which means rehearsing a total of 256 songs), culminating in their new album Exotic Creatures of the Deep, a record which proves that, while the world was intermittently paying attention, Sparks have never let their quality control dip below excellent.

Some fans with a "golden ticket" are attending every single gig. The rest of us have to be choosy. For my first visit, I pick Indiscreet, the 1975 album which was a classic case of commercial suicide equalling artistic triumph: its cover photo, depicting the Maels crawling from the wreckage of a light aircraft, says it all. Having fleetingly become pin-ups to the Jackie generation with "This Town Ain't Big Enough For Both Of Us", Sparks baffled their teeny audience with an album informed by Charleston swing and Weimar cabaret rather than rock'n'roll, produced by Tony Visconti at his most berserk, and with lyrical themes ranging from the disfigurement of a Parisian hotelier in a terrorist blast to the desexualisation of breasts after childbirth.

The album only reached No 18, and would be their last in the UK Top 20. For its live resurrection, in keeping with its musical mood, Russell looks fantastically 1920s in Prohibition pinstripes, and Ron, his necktie longer than his waistband, also looks immaculately proper, in a pre-war sort of way. And they push the boat out on such classics as "Get in the Swing" and "Looks Looks Looks" with a brass quintet, a string sextet and, brilliantly, a school sports whistle whipped out of Russell's pocket.

They're as charismatic and potent as ever. As they accept the acclaim that follows, Russell describes this residency as "a preposterous idea ... that you've made a little less preposterous". Which is as good a description of Sparks as one could wish for.