L'incoronazione di Poppea, Glyndebourne <br/>Roméo et Juliette, Theatre Royal, Nottingham<br/>Labyrinto, St John's Smith Square, London

Don't blame the singers &ndash; they, and the orchestra, are ravishing; it's the staging that kills this Monteverdi opera
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Some say its subject is lust, others say power. But L'incoronazione di Poppea is about lies. The little lies that protocol demands, the big lies of politics, the half-truths whispered by lovers in the throes of desire, the falsehoods told by those who are prepared to die for passion, the self-deception maintained by those who recognise too late that they have fallen for a monster. Arias are rare in Monteverdi's last and most ambiguous opera. For nearly four decades after the premiere of L'Orfeo, with its richly decorative writing, the first great opera composer returned to first principles, turning speech into song in the sparest, most sophisticated of forms: recitative.

Sombre, sensual, saturated with perceptive detail and directed from the keyboard by Emmanuelle Haïm, the unhurried performance of the Orchestra of the Age of Enlightenment's expanded continuo section (three theorbos, lirone, violone, viola da gamba, cello, organ, harp and harpsichord) is the only aspect of Robert Carsen's theatrically muddled Glyndebourne production to prioritise the libretto.

It would be unfair to blame the singers, for there are errors here that would be surprising in a student production: placing the lightest voices so far upstage that their connection to the orchestra is compromised; swathing the set with acres of voice-extinguishing velvet; exposing the wings so that any sound not sucked up by the fabric travels sideways.

Presented with a score of striking intimacy and subtlety, it seems that this highly experienced director has panicked. The death of Seneca (Paolo Battaglia) barely registers, yet the prologue is preceded by a jejeune skit in the auditorium. The supper interval is signalled by an onstage picnic, and the closing image of the titular courtesan sees an entire chorus and several dozen chairs collapse under a tsunami of cloth.

In contrast to the acid-brights of Chen Shi-Zheng's Poppea for ENO, Michael Levine's designs are drab and crepuscular. Three panels of dull red, floor-to-ceiling curtains wheel around the stage, forming the walls of Poppea's boudoir and the corridors of power where Nerone fornicates and fulminates, sometimes on a bed, sometimes in a bath. Sumptuously, dangerously, ravishingly sung by Alice Coote – with every dissonance squeezed and stretched to thrilling effect, every vocal colour explored – Carsen's Nerone is nonetheless a total bore: sociopathic from the tips of the fingers to the toes that we know he will later use to kick Poppea to death.

Dressed unflatteringly by costume designer Constance Hoffmann, Poppea herself remains an enigma. Voluptuous and wide-eyed, Danielle de Niese is at her best in the long Act I sequence with Coote, but lacks the expressive range to nourish Monteverdi's low-pitched, deceptively simple lines. She also lacks the acting ability to convey mixed motives.

Among the large supporting cast, several performances stand out: Tamara Mumford's vivid, bitter Ottavia, Marie Arnet's ardent Drusilla, and the inimitable Dominique Visse, whose sinus-clearing Nutrice pitter-pats across the stage in a Margaret Thatcher suit and pussy-bow blouse. Amy Freston's gamine Amore – a near-constant presence in this production – shows poise and polish, as does Christophe Dumaux's slender Ottone, when he is not swamped by the soft-furnishings. From the ecstatic flourishes of the cornets to the mournful sighs of the lirone and gamba, the golden seam of lute and harp, and Kati Debretzeni's exquisitely decorated violin solo, I will count myself lucky if I ever hear a better-played Poppea. Theatrically, however, this is a poor start to Glyndebourne's season.

There must be times when today's opera singers yearn for the days of park-and-bark direction. In Carsen's Poppea, the cast contend with acoustic-wrecking curtains and cold baths. In John Fulljames's taut, touching production of Roméo et Juliette, Leonardo Capalbo and Bernarda Bobro are required to canoodle while balancing on a tilted platform suspended on wires some 10ft above the stage, mostly in their underwear. I do hope Opera North is paying them well.

The third production in the company's Shakespeare season, Roméo et Juliette uses the same basic set as Macbeth and A Midsummer Night's Dream, now clad by designer Johan Engels with (voice-friendly) panels of reflective black. The look is part swimming-pool, part-mausoleum, part-night-club: an unforgiving setting for the tender twosome to fall in love, but a great place for Frances Bourne's saucy Stéphano to spray graffiti. Young-looking and, in Bobro's case, young-sounding, the star-crossed lovers have an innocence to match their vocal agility, and though some of Adam Wiltshire's mafia-meets-LA punk costumes show an obvious debt to Baz Luhrmann, Peter Wedd's Tybalt, Stephan Loges's Mercutio and Yvonne Howard's Gertrude are equally well drawn. Under Martin André, the orchestra and chorus deliver an impassioned and impressive account of Gounod's grave, opulent score.

I closed this week with the heavy-lidded sound of the viola da gamba. Paolo Pandolfo's late-night Lufthansa Festival recital of music by the 17th-century soldier-composer Tobias Hume unlocked a world in which melancholy was celebrated. This was a world in which gentleman amateurs could indulge what Hume described as "the effeminate part of mee" (his love of music) through dances and laments for two or three viols. Together with Labyrinto's Guido Balestracci and Rebeka Rusó, Pandolfo's nimble divisions, broken chords and breathtaking pizzicato were a beguiling nocturnal pleasure. Perhaps I'm jaded, but it seems unlikely that the music of James Blunt will last as well.

'Poppea': Glyndebourne (01273 813813) to 4 July. 'Roméo et Juliette': Theatre Royal, Nottingham (0115 989 5555) 30 May, then touring