Death Cab for Cutie plays Koka Booth Amphitheatre at Regency Park Monday, Aug. 8. Tickets for the 7 p.m. show are $32–$37. Frightened Rabbit opens.

Death Cab for Cutie needs a new singer. This isn’t meant to disparage Ben Gibbard, the sensitive-voiced frontman who began the band as a solo project in the late ’90s. No, Gibbard is a fine vocalist, generally capable of adding the proper touch of hurt or sweetness to his understated coo. And from Death Cab’s “Title and Registration” to “Clark Gable,” by Gibbard’s electronic pop side-project The Postal Service, he’s turned in several unforgettable performances during the last decade. It’s just that, if Death Cab for Cutie want to continue expanding their sound and approach, as their new album suggests they do, his slight sound is going to need some support. Whether it’s back-up singers or another member occasionally taking over lead vocals, he’ll need help singing in order to keep up with Death Cab’s consistent evolution.

Death Cab for Cutie began as a mostly typical indie rock bandguitars, drums, bass, some keyboards and a frontman, playing songs that were mostly about love or the general social troubles of being a handsome young American band. Thanks to a few good marketing moves, like song appearances on The O.C., and the overwhelming success of The Postal Service, the band’s trajectory shifted toward the mainstream. In 2003, Death Cab made clear on Transatlanticism that it was interested in being more than a band with the credo “You Can Play These Songs with Chords,” the title of Gibbard’s first demos as Death Cab. The next major step came in 2005, when they released Plans, their fifth album and first for Atlantic Records, a major label that for the last decade has been keen on pushing indie acts toward the masses. Plans featured the glimmering, harmony-rich jangle of “Soul Meets Body,” the piano-into-noise movement of the heartrending “What Sarah Said” and the post-rock highs of “Marching Bands of Manhattan.”

Around the time of that album’s release, Chris Walla, the band’s longtime multi-instrumentalist and producer, commented that he felt as if he were finally starting to learn how to make records, how to produce, how to do the job he’d ostensibly been doing for the previous several years. That idea was a surprising one from someone who’d already sold a lot of records and who was on his way to selling a lot more, but as Gibbard told me in May, he still thinks of Walla as an incessant tinkerer who’s growing as the band ages. “Whenever I’m away from Chris for some time and he’s been working on records, I come in and there’s always something new that he’s selling us. He’s got some set of equipment that he’s learned how to use and he’s really inspired by,” Gibbard said. “Because he’s such a passionate musician and so dedicated to continuing to push his own envelope, I think the sky’s the limit for what he’s able to do as long as he remains invested in it.”

You can indeed hear that expansion on this year’s Codes and Keys, Death Cab’s third for Atlantic. “Unobstructed Views” floats in an electronic web that shimmers and finally shudders, like Amnesiac-era Radiohead recalibrated for the radio. “Doors Unlocked and Open” rides a rigid Motorik beat, while closer “Stay Young, Go Dancing” seems but a few touches away from modern country affectation. Throughout it all, though, Gibbard’s voice is a monochromatic pastel, dependent on effects to make his tone interesting at all in “Unlocked,” to make himself sound tough amid the distorted-bass rumble of “Some Boys.”

From Modest Mouse’s Isaac Brock to Bright Eyes’ Conor Oberst, plenty of other leaders of indie-gone-mainstream acts have found ways to adapt their singing, to fit changes in the band’s sound while still sounding like themelves. But Gibbard simply lacks the versatility to lead Death Cab’s continually broadening palette, meaning he sounds exactly the same from song to song even though the band’s arrangements shift dramatically. That’s where the otherwise genuinely interesting Codes and Keys feels like a, well, stylistic dead end for Death Cabproof that all the new production interests in the world can’t overcome a leader locked in his vocal ways.