If there's operatic box-office insurance in Philadelphia, it's not the latest hot Estonian mezzo or some vocal legend coming out of retirement to sing one last

Aida

- it's a medium-famous tenor/baritone pairing that looks as good as it sounds: William Burden and Nathan Gunn. And if ever they were needed, it's in the searching, provocative Britten opera

The Rape of Lucretia

, opening tomorrow in the finale of the Opera Company of Philadelphia's season.

Ever since their simultaneous breakout in a famously shirtless production of Iphigenie en Tauride at Glimmerglass Opera in 1997, their careers have had enough Butch-and-Sundance intersections that a conservative operagoer might think they rarely sang apart.

They wouldn't mind that.

"It's so easy. There's a very natural level of trust onstage," says Burden, the blue-eyed, fair-haired tenor.

"That goes a long way with me," says the dark-haired, dark-eyed Gunn, "to have a rehearsal process where you can try a lot of things out."

"You want to be able to be really wrong," adds Burden, "and have a colleague you can have a drink with afterward and laugh about how wrong it was. . . ."

"But he's always trying to keep me away from his wife," complains Gunn, 38.

"My kids have brown eyes," says the 44-year-old Burden, "which I always felt funny about."

In Philadelphia, the two have been heard together in Cosi fan tutte and The Pearl Fishers. Though they both have fine roles in The Rape of Lucretia, the opera keeps them in separate spheres: Britten's third opera, which turns up in Philadelphia every few years, is a parable-style story, told with excruciating candor, of an Etruscan prince who rapes a highborn Roman woman of unimpeachable integrity. Narrating and commenting on the story are solo male and female singers who comprise the story's conscience, looking back at it from a later Christian standpoint.

Gunn plays the rapist and Burden the male narrator in a bit of art-imitates-life casting: the Miami-raised Burden is the brainier of the two (he graduated from Middlebury College with a degree in Spanish literature) and is dressed as a dignified, modern-day evangelist. The South Bend, Ind.-born Gunn came out of the University of Illinois and, though a perfectly adept recitalist, is the kind of stage animal who doesn't settle for a single dimension: His rapist is deeply in love with his victim and is emotionally ruined when the opera is over.

Though not the easiest box-office sell, the opera was chosen as OCP's first solo production in the Kimmel Center's smaller venue for several reasons: It's the kind of challenging repertoire OCP wants to present there in future years. And the principals had sung their roles previously, Tamara Mumford (Lucretia) having done so only last year at the Lorin Maazel-founded Castleton Festival in Virginia, from which OCP rented the sets.

The Perelman dates intentionally fall after regular opera seasons and before the summer festivals, says OCP executive director David Devan. "When you hit a zenith in your career, you start looking for interesting gigs, things that help you as an artist and keep you connected to your craft." Gunn wistfully wonders if he'll ever have a chance to sing the opera again.

Both singers are accepting lower, chamber-opera fees. Though production costs are half those of any Academy of Music endeavor, five performances in the 650-seat Perelman Theater have only 25 percent of the Academy's sales potential. But in the Perelman, singers have the luxury of not pushing their voices and, as a result, can project words so clearly that surtitles might be nearly superfluous. Also, a shorter, more intensive rehearsal period is possible with fewer characters.

"Both of us support the development of this chamber series," says Burden. "It can draw the audience into a more hands-on, intimate experience of the art form."

"It breaks down barriers that have been built over the years," says Gunn. "I love the libretto. It's typical of Britten in how it discuss the nature of what happens to goodness in the world, and how it's corrupted by our touch."

"It's a piece people will be forced to discuss as they leave," says Burden. "And because I live in Princeton, I get to tuck my kids in at night."

Burden has two. Gunn's five are in Champaign, Ill., where he and his wife, Julie, are on the University of Illinois faculty. Their whole brood came to Philadelphia a few years ago when Gunn performed the song cycle Die Schöne Müllerin here; this time, only Gunn's 14-year-old daughter, the eldest, will be allowed to see what he's up to in Philadelphia.

"I'm kind of choosy about what operas I let my kids see. This one might not be appropriate," he says. "When we did Iphigenie [at Glimmerglass] we had to fight onstage, and Madeline . . . got very upset watching us fight. She had only seen us hanging out, having barbecues, watching Disney videos."

That Iphigenie had the two protagonists surviving a shipwreck with their clothes barely intact. Both headed for the gym; Gunn inspired the word "barihunk." Watching how their paths have diverged since then is an intriguing case study in career options.

Both voices find happy homes in The Magic Flute and Cosi fan tutte. From there, Burden's appearances at some of the big houses have been in slightly left-of-center works such as Debussy's Pelleas et Melisande at the Met and Martinu's Julietta at the Paris Opera. Gunn sings much contemporary opera. Only weeks ago, he premiered Andre Previn's Brief Encounter in Houston. Last year at Glyndebourne, he played a Jesuit priest who falls in love with the girl he's exorcising in Love and Other Demons by Peter Eotvos.

Both have veered toward musical theater, but in very different ways. Burden was in such a wildly irreverent production of Bernstein's Candide at Paris' Chatelet Theater that the subsequent dates at La Scala were temporarily canceled until the director agreed to tame it all down. Gunn made his own disc of pop songs for Sony, titled Just Before Sunrise. Recently, he hired Placido Domingo's high-powered publicist. Crossover, anyone?

"Crossover, in my mind, means doing something you shouldn't," he says. "In all the operas I've done, I've never had an audience respond as long as when I did 'If Ever I Would Leave You' " in a New York concert performance of Camelot. "I don't think they'd ever heard a [operatic] baritone sing it. It's not a hard song. It's just a beautiful song. Is that crossover or what we should be doing?" He's considering a short Carousel run at New York's City Center.

Britten brings the two singers back together. Though audiences will puzzle over everything from the nature of the Christian narration in Lucretia to the sense of the composer's word settings, Gunn and Burden talk about the pleasure of dealing with a composer who knew just what he wanted and wrote it down with complete precision.

"If you just try to get it right, the experience will take care of itself," says Gunn. "All you have to do is be a conduit."

Says Burden, "It's all there."

Contact music critic David Patrick Stearns at dstearns@phillynews.com.