It was the promised coup of this year's Festival: Bryn Terfel easing into the supreme bass-baritone role, Wotan, which the whole world knows him to be destined for. His first stab at The Big One, and a major entry on the Edinburghian record of you-heard-it-heres.

Except that you didn't. Terfel cancelled. And so, from the number of empty seats, did a proportion of the audience for whom it was Terfel or nothing. But they were wrong, because Thursday's concert performance of Die Walkure Act III at the Usher Hall still had Jane Eaglen as its Brunnhilde; and with John Tomlinson - Wotan to Bayreuth and Covent Garden - as the most illustrious of substitutes, it proved a truly great occasion.

There's a lot to be said for isolating a single act of Wagner. The text comes into closer focus; sensitivity replaces volume. And I've never heard Jane Eaglen in such radiant voice: every note the product of an artist whose repertoire is bel canto as well as Wagner heroines, brighter than most Brunnhildes, with less evidence of the mezzo-ish foundation they usually need to sustain the sound.

We don't hear Eaglen often in this country. Since she made her name as Scottish Opera's Norma and Brunnhilde in the early 1990s, her career has shifted to America where - with exeats to Vienna and La Scala - she has joined the youth division of leading contemporary Wagner singers. John Tomlinson's Wagnerian credentials need no elaboration, except to say that as a true bass rather than bass-baritone, his strengths lie in Wotan's calmer and accordingly lower-pitch moments rather than in the elevated, tone-thinning fierceness that much of Act III demands. His "Farewells" were entrancing, rich as syrup in a huge, dark vat.

And though we were denied the discovery of Terfel in Wagner, there was a real find in Antonio Pappano, the British-American conductor whose reputation here has so far been based in Puccini. From an uncomfortably foursquare start, with a "Ride of the Valkyries" that seemed to need another shilling in the meter, it emerged that he was able to generate intense excitement from the sheer control of these slow tempi. His command of detail was exemplary, and the Scottish National Orchestra played as though their lives depended on it.

The conspiracy to displace opera companies from their homes and send them out on the streets has claimed another victim: Broomhill, the alternative country-house opera co which, after only four seasons of life on the Kent estate from which it takes its name, was summarily evicted earlier this year. The owners wanted the space for less exalted entertainments (disco-dancing) and Broomhill's management were on the brink of sorting out their sales pitch for The Big Issue when Christ's Hospital, the school near Horsham, offered them its on-site theatre. So the company is back in business and has managed to pull together a fifth season with something of the raunchy, firing-from-the-hippish chutzpah of its previous ones. "Love, Lust and the Lottery," declare the poster ads, in a determined attempt to take the appeal of opera beyond Glyndebourne niceness. And the Love and Lust refer, I suppose, to Rossini's Turco in Italia, which is the first big show of the season - although this production doesn't quite deliver the steamy romp you're led to expect.

Turco is an oddly futuristic piece of theatre: conventional enough in its basic plot, but remarkable in the way events unfold through the mediation of a narrator-poet who appears to be writing the story as it happens. Rossini never put his hand to anything more obviously experimental - it could be Pirandello before his time - and the whole thing is a gift for directorial imagination.

The pity is that Simon Callow, the director here, makes little of it. His production is conservatively decorous, short on fizz, and undervalues the sharpness of the irony that fuels the laughs. But that said, a pleasing feelgood factor keeps everything afloat. It's well-designed, with a spectacular ship to carry the Turk to Italy (the sort of thing you normally find flying Wagner's Dutchman to Norway). And there are some nice performances, led by an outstanding Fiorilla in Marguerite Krull: a Canadian singer of impressive presence with a well-defined but silken coloratura. One of the composer's favoured breed of feisty mezzos, Fiorilla was a Callas role: half-tragedy, half-comedy, and a demanding sing, especially in the dry acoustic at Christ's Hospital. But Krull copes grandly and heroically. She makes the show.

Much of the past week at the Proms has been given over to Americana, starting with the Dallas Symphony Orchestra, which lived up to the image of its home town with a forcefully assertive strength, a handsome brass sound and the musical equivalent of a JR Ewing swagger. But under its new music director, Andrew Litton, there were also signs of an orchestra which has learnt to listen critically to itself. It made ravishing work of Samuel Barber's Violin Concerto and had a wonderful soloist in Joshua Bell, who has come far since those early days when Decca tried to launch him as a heart-throb. He performs with physically demonstrative conviction. And amazing stamina.

Twentieth-century American music has been preoccupied with a pursuit of identity, edging away from European models; and Tuesday's Prom, played by the BBC Symphony Orchestra under Oliver Knussen, considered two of the approach-paths: Aaron Copland's open-hearted, cinematic prairie music as against the cerebral particularity of Ruth Crawford Seeger (in the 1930s) and Elliott Carter (now). At face value they haven't much in common. But as this programme showed, the interests of the prairie men and cerebrals did sometimes overlap, not least in the artful setting of vernacular folk songs. And allegiances have changed with time. Elliot Carter in the 1940s was writing cheerfully Coplandesque Americana like the Holiday Overture that opened this programme. Only a considered reading of the way its rhythmic complexities subvert an otherwise breezy, Big Country robustness could prepare your ear for the completely different world of his latest work, Allegro Scorrevole, which had its British premiere later in the evening. The third instalment of a symphonic triptych, Allegro Scorrevole is an essay in the abstraction of delicate complexities from large resources, constantly in movement, like a Heraclitian fire but not so hot. In fact, the emotional temperature of this music is deadeningly cool. Like most of Carter's recent work, it commands respect as a display of mental agility (astounding for a man now in his 90th year) rather than engagement.

In contrast to that, you'd have to tear me away from the other new work in this Prom: Mark-Anthony Turnage's Dispelling the Fears, which was written in 1995 but was receiving here its London premiere. A quasi-concerto for two trumpets (John Wallace and Hakan Hardenberger), it has a documentary quality: evocative of urban desolation, nights alone on empty streets. The brass writing is blues-seductive, and with Ivesian Unanswered Questions somewhere in its cultural antecedents. I doubt if Turnage has ever written anything so beautiful or so profoundly moving - qualities which surface rarely at the hard edge of contemporary music. Warmly played, instinctively conducted, it seemed made for the resounding spaces of the Albert Hall. Not everything you hear during the Proms could make that claim.

'Turco in Italia': Newcastle Tyne Theatre (0191 232 0899), Tues, Thurs & Sat.