It's easy but not accurate to call Tenacious D a one-joke band, since they do love one joke best of all: that they are the greatest band in the world.

It's a credit to Jack Black and Kyle Gass' strengths as writers and performers that at their best they can convince you it's true.

Like the best comedians, the key is both in the writing and the delivery: jokes can be good on paper, but they need to be delivered with flair, and few have the flair of Jack Black, who has made megalomania inspiring, even adorable.

That quality combined with serious vocal chops -- anybody who saw him on Mr.

Show's "The Joke: The Musical" back in 1997 knew that he could sing -- gave Tenacious D both star power and musical substance, while Gass grounds it by giving Jack a comic foil, plus lead guitar and harmony.

When it all gels, as it did on their short-lived HBO series and their 2001 debut, it's glorious, but even that 2001 LP indicated a problem with the D: when the scale gets larger, they get smaller, or at least their reason for being begins to unravel.

Since the reason their joke works is that JB and KG are underdogs -- they're the best band in the world, it's just that the rest of the world hasn't figured it out yet -- when they're no longer underdogs, they're not quite as funny, or endearing.

They're at their best when it's the two of them on-stage, playing acoustic guitars and riffing off each other.

They're good enough that they can survive a bigger budget, as the debut illustrates -- it always helps to have Dave Grohl on your side, of course -- but a really big budget is still a problem, as the soundtrack to their big-screen extravaganza, The Pick of Destiny, proves.

Jack and Kyle have been promising a cinematic venture chronicling their rise to power since they -- alright, since Jack turned into a star after stealing the show in the 2000 film High Fidelity, and 2006's The Pick of Destiny, made in collaboration with director/musician/prankster Liam Lynch, finally follows through on that promise.

Leave aside the merits of the movie and compare the The Pick of Destiny soundtrack to the debut, and it's easy to see that this album is a very different beast than Tenacious D.

That first album captured the essence of the original D -- the D that was nothing but Kage and Jables and their guitars -- but pumped up with heaps of electric guitars and thunderous drums from Grohl.

It cribbed from a lot, but not all, of their standards, so it felt like a culmination of sorts: it finally felt like the D blossomed into a genuine rock band.

In turn, The Pick of Destiny has greater ambitions -- appropriately for a soundtrack, it's big, sweeping, and well, cinematic -- but it doesn't feel like a breakthrough, since Tenacious D already took the D just about as far as they could go musically: it gave them muscle and might, it fleshed out their skeleton, sometimes a little bit too much, yet it worked because it sounded like this must be what JB and KG heard in their heads when they played on their own.

There's no difference in sound on The Pick of Destiny, but the aesthetic of a soundtrack makes a huge difference.

This may not be a concept album, but it's structured as a narrative, mirroring the plot of the movie.

Unfortunately, this doesn't give The Pick of Destiny the weight or grandeur of a true concept album, because a lot of the music sounds as if it serves the movie, and doesn't stand tall when separated from the film.

It's easy to figure out that songs like "Break In-City (Storm the Gate!)" and "Car Chase City" are plot points in the film, but it's not quite as simple as that: since most of the album consists of songs that run between 1:20 and 2:40 minutes, all the tunes kind of feel like narrative filler, even when they're melodic, memorable, and delivered with gusto by the D.

And that's the crucial problem with the album: it's good, but it doesn't have the surplus of songs so great they sound like unearthed classics, which is the very thing that has always made Tenacious D so irresistible.

Make no mistake, they're still great enough to rally: they revive "History," their indelible theme, incorporate "Sasquatch" into the deliriously atypical psych-pop "Papagenu (He's My Sassafrass)," offer a Dethklok-worthy ode to metalosity with "The Metal," and serve up two epics in "Beelzeboss (The Final Showdown)" -- served up as a duet with Dave Grohl, who plays Satan -- and the opening "Kickapoo," a tremendous mini-rock opera with cameos from Meat Loaf and Ronnie James Dio.

Excellent moments, but it doesn't add up to a record that's as satisfying an album as the debut.

This is a bit disappointing, but The Pick of Destiny is good as a soundtrack: a souvenir for fans of the film.

That's enough for some portions of the legions of D-heads, but for some who have long loved the D, it's hard not to hear The Pick of Destiny and wish that it rocked both of your socks off instead of just one.