With Nick Ashford and Valerie Simpson, a sigh's not just a sigh. This songwriting couple personifies the essence of romantic grace, both in their silk-draped, ultra-suave visual presentation and their finely meshed sophistication-on-ice vocal performances. These elements work for, as well as against them on "Street Opera," their most recent release.

Side 1, though the less ambitious, is a competent and considered collection of typical Ashford and Simpson paeans to adult love. If on their own this duo never soars to the rarified altitudes reached in songs they composed for others ("Ain't No Mountain High Enough," "You're All I Need to Get By"), these songs find them gliding smoothly through tasteful arrangements and high-gloss orchestrations.

"Love It Away" is the album's finest cut. Ashford's smokey voice evokes a mature compassion. It's the perfect foil for Simpson's honeyed, wide-eyed vocal style, and the promise of the chorus to "love it away / so cheer up" is convincing and infectious.

"Make It Work Again" offers a soulful buildup of romantic tension and dynamics; a quick comparison with its topical second cousin, Peaches and Herb's "Reunited," reveals how wide is the gap of sophistication, both lyrically and musically, between the two duos. And Simpson delivers "I'll Take The Whole World On" with conviction, letting its swaying, stately progression swell dramatically without ever allowing it to tip over into melodrama.

The first side lays a vague thematic groundwork for the side-long title track, and it's here that Ashford and Simpson begin to dip and bobble stylistically. The tale of a doomed ghetto romance in five parts, "Street Opera" concerns a man unable to withstand the economic and social pressures closing in on him until finally, he feels forced to let woman and children fend for themselves while he attempts some undefined goal.

The third movement is the only one in which the music matches the message. The song maintains its integrity through hard- knocks lyrics and soft-rock approach. The reprise of "Working Man" blossoms in a polished, lush-toned finale.

But "Who Will They Look To" is sappy, drawing on elements of sexism and irresponsibity that damage the work as a whole. Simpson's handwringing over the plight of the children is unconvincing, if only because it plays to too many sterotypes at once.

But the real credibility problem is that after years of hearing Ashford and Simpson define the pleasures of refined and graceful romance, it's hard to picture them in this gritty, down-and-out scenario. With a wispy and ill-defined story, they seem to be trying to express the same kinds of sentiments they've already limned successfully and strongly on side one. On "Street Opera," styles collide and come dangerously close to nullifying each other.

Style isn't everything, but Ashford and Simpson write and sing with such attractive and eloquent style that when they're anything less than beautiful, they're hardly themselves at all.

ON RECORD, ON STAGE THE ALBUM -- Street Opera (Capitol ST 12207). THE SHOW -- Ashford and Simpson, Friday and Saturday at 8 and 11 at Constitution Hall.