Part of the risk with Theodora is that it isn't actually an opera but an oratorio; and although Handel's oratorios are sometimes staged, they were never meant to be: their choruses are generally too big and static, their story-telling too perfunctory. But Theodora - a martyr's tale about the virginal Saint Dorothy who deems death a better fate than sex-slavery in the temple of Venus - does lend itself to theatre, albeit of a highly formal kind. And the very absence of a stage tradition means that Peter Sellars has nothing to violate as he did with his notorious Glyndebourne Magic Flute. That was booed on its first night. Theodora was cheered - rightly, because it's good: riddled with regrettable incident, perhaps, but with an underlying intelligence and musicality that wins through.
On the down side, it clings to Sellars's pernicious belief that opera only speaks to modern audiences if it's saturated in American vernacular - so Theodora's Roman governor becomes John F Kennedy with a drink problem, and our old friends the Men in Combat Outfits with Machine Guns do their routine. And for anyone with less than fond memories of the way the Flute singers sculpted their music in corresponding sign- language, I should add that they're still at it. No doubt the idea is to reinvent baroque gesture and intensify the emotions, but the result is the opposite. Passion dilutes to artifice.
On the up side, though, Sellars has coaxed performances of great eloquence from his cast, one of those lean, refined and flexible ensembles that exemplify the modern American approach to opera. No bluster, no excess baggage, just focus, commitment and Midwestern innocence: notably from Dawn Upshaw, undercast in the title role but touchingly vulnerable and pure of sound. There is also an exceptional American counter-tenor, David Daniels, who takes the castrato role of Didymus with a gentle, creamy, hootless lustre: one of the loveliest and most "finished" voices of its kind I've ever heard. But the star is Lorraine Hunt, the much-heralded American coloratura mezzo who makes her British stage debut as Irene. Rich in timbre, she has style, precision, presence, lyric beauty - all the attributes the ear could ask - and a limitless future. After this, the opera managements of Europe will be at her feet.
As for William Christie, it's disconcerting to find him grappling with period issues in the pit (the band is the Age of Enlightenment Orchestra) while beer cans and machine guns rattle on the stage, but it all somehow comes together, elegantly phrased and unfailingly musical. The text suffers from his apparent instruction to the chorus to kill their tone before the final consonants of sustained syllables - "He saw the lovely youth" becomes "He saw the lovely ewe", an interesting variant - but otherwise I've no complaints. Theodora is a night of glorious music gloriously done; and yes, it even looks good in the spare white box of George Tyspin's set. When the curtain rises on rows of chairs and giant clones of ancient artefacts, you think of Nicholas Hytner's Xerxes; but it turns out more like Robert Wilson, a compound of the clinical and lyrical worked into starkly lit but stunning tableaux. If this is the New Glyndebourne, I'm all for it.
A bit of the old Glyndebourne survives with a revival of Trevor Nunn's Dames-at-Sea Cosi fan tutte. Although the idea of Cosi as a cruise-ship romance holds good, the West End brashness is wearing thin enough to suggest it's time this ship was honourably scuttled. Monday's opening night was enjoyable but not quite Glyndebourne-standard; the essential balance between the main voices was lacking. Simon Keenlyside's Guglielmo and Susan Graham's Dorabella were fine, but Solveig Kringelborn's big-boned Fiordiligi didn't fit, and John Mark Ainsley was in difficulties as Ferrando. Franz Welser-Most was conducting the LPO, and shouldn't have been. From the first bars my heart sank to hear how unstylish, leaden and rhythmically choppy it all was. This may be a marine production, but Mozart is not verismo. You don't need to feel seasick.
Welser-Most should spend some time at Covent Garden's revival of Die Entfuhrung and hear the mastery Colin Davis brings to Mozart. It's a mixed- quality cast, led by Eva Mei whose coloratura is bright and accurate but more like an alumnus of the Vienna Boys' Choir than a true Konstanze. Kurt Rydl, Kurt Streit and Inger-Dam Jensen provide more wholly pleasurable performances. But Davis's conducting is the big attraction: Mozartian to the nth degree, accomplished and benevolent. The music glows under his beat; and for that, the odd audible groan as he sings along is a small price to pay.
When the Mastership of the King's Music fell vacant in 1941 it went not to Vaughan Williams or Walton but to Arnold Bax, whose seven symphonies had earned him a status that tends to be forgotten these days. He has become a shadowy figure: one of those Celtic dreamers who belonged emotionally if not technically to the 19th century. But Chandos records has been keeping his name alive with an expanding discography; and Christ's Hospital School in Sussex has just hosted an ambitious Bax Weekend devoted to his chamber music and featuring artists like the Maggini Quartet, Nicholas Daniel and John Hancorn. The reason was local loyalty: Bax lived nearby for the last 12 years of his life. But as the weekend's excellently put-together programmes proved, his heart and mind were elsewhere: partly in Ireland, whose legends fired his imagination; partly in the Franco-Russian alliance of the Ballets Russes, whose composers (especially Ravel and Debussy) coloured his harmonic language. Called on to be the English patriot - in his score for the wartime propaganda film Malta GC, shown during the weekend - he never made the grade, producing a half-hearted Waltonian pastiche that Walton would have done better himself. Perhaps, with hindsight, that was Bax's endearing quality: his failure to be what was expected, his embryonic subversion. He may have more to offer the late 20th century than we thought.
'Theodora' and 'Cosi fan tutte': Glyndebourne (01273 812321). 'Entfuhrung': ROH, WC2 (0171 304 4000).