OPERA / Hogging the ball: Verdi, revived but unwell in Cardiff: Un Ballo in Maschera - WNO, Cardiff

Is Un ballo in maschera Verdi's most brilliant opera or his silliest? The answer may well be both. I don't myself think that Goran Jarvefelt's Cardiff production, revived on Monday, makes it any more 'sensible' by transferring it back from Boston (where the censor made Verdi set it) to Stockholm, where Gustavus III's assassination actually took place. But what's clear is that Ballo needs fluent singing if its musical brilliance is to carry the drama, as the best Verdi critics have always maintained it should. And at the moment Welsh National Opera achieve this only in patches.

What Jarvefelt and his designer Carl Friedrich Oberle get from the Stockholm connection is visual charm and atmosphere: some beautiful painted drops, sparkling costumes with a touch of frontier grotesque, sharp northern light, and with it sharp portraiture. I've less patience with the funny business of Gustavus playing with his doll's house stage right, or with the inevitable producerly sideswipes at monarchy. Malcolm Hunter's revival of the 10-year-old production still has its creaky moments, but also plenty of heart and intelligence. With time, and a spot of oil, it should belie its age.

Vocally, though, the revival needs a doctor more than a mechanic. On Monday, Donald Maxwell could barely penetrate a chest infection to make anything of Ankarstrom's two crucial arias, and the effort (I suppose) drove him into gestural exaggeration. The Oscar, Rebecca Evans, was a less obvious sufferer, though also plainly below her best. She frisked energetically in 'Saper vorreste'; but the full vocal equivalent was not always there.

These two recoveries will be well worth waiting for. The Canadian tenor Richard Margison's Gustav is worth hearing already: a splendidly vital and personable reading, which catches well the sententious frivolity that is the character's doom. There are touches of coarseness in Margison's singing; but they pale beside his consistently alert musicality. And he gets better. 'Ella e pura', delivered from the floor and with a bullet in his chest, is as controlled as 'Dunque, signori', and as touching as his gallows love-song 'Non sai tu' - passion with a boyish twinkle.

Lisa Gasteen's Amelia is less completely convincing. She can be thrilling when challenged. But she sounded ordinary in the middle register of 'Morro] ma prima in grazia', and was more stately than distraught in the gallows scene. Anne Marie Owens, similarly, is a subdued Ulrica, but also an intelligent one; she played nicely the subtle confusion of the handshake scene (struck, of course, by the encounter with a future regicide).

Carlo Rizzi conducts, as ever, with terrific style and unwavering spirit. The orchestra and chorus sparkle. Some of Rizzi's tempi took the singers by surprise. But this, like many of the other minor troubles, will surely vanish with the morning mist.

Further performances tomorrow and 4 March, Cardiff New Theatre (0222 394844). Then tours: details on 0222 464666