When Nickel Creek plays to a hometown crowd Dec. 15 and 16 at House of Blues San Diego, the North County-born band will do so newly blessed with two Grammy nominations related to August's "Why Should the Fire Die?"

Bluegrass figures to be present with Chris Thile's mandolin reaching dizzying heights where few can breathe. Such heat will probably be tempered by shaded chamber pop reflections, then rekindled on new ventures that stray closer to indie rock than Bill Monroe.

There's bluegrass —— two instrumentals in particular, the Grammy-nominated "Scotch & Chocolate" and "Stumptown" —— on "Why Should the Fire Die?," but the album often sounds like one from a band that has shed its skin and started fresh. There's the tweaked old-timey skip of "Anthony," the tension-swelling "Helena," which may have you reaching for your Arcade Fire album by the time the song erupts into a cacophony that includes co-producer Eric Valentine pounding the skins. Yes, drums.

"It just sounds like what it is," guitarist Sean Watkins said. "We never thought, 'This is going to sound sorta rock.' We don't really think about that. It just kinda evolves."

The band has had plenty of time to cultivate its sound. Mandolinist Chris Thile, 24, and the Watkins siblings, six-stringer Sean, 28, and fiddler Sara, 24, have been playing together for 16 years. The group has followed-up a self-released CD with three on acoustic/bluegrass label Sugar Hill. Each of the three proper albums has garnered two Grammy nominations, with "This Side," winning for Best Contemporary Folk Album. Both Thile and Sean Watkins have multiple solo records to their credit, with one planned by Sara Watkins. Sean Watkin's latest CD, "Blinders On," comes out early next year but can be downloaded (for a fee) on his Web site.

The band members, whether together or separately, have collaborated often with other artists, including as a unit with former Toad the Wet Sprocket frontman Glen Phillips under the banner Mutual Admiration Society. The foursome released a CD last year, although they recorded it in 2000. When not on tour or in North County, Watkins said he and his sister have been living part time in rented spaces in Los Angeles, with Thile making his presence known, too.

"All of my favorite musicians live there and there's an unbelievable music scene that I've gotten to be apart of," Watkins said.

Their time in L.A. has included sitting in with such respected musicians as uber-producer/arranger Jon Brion and Benmont Tench, who may be best known as one of Tom Petty's Heartbreakers.

"(Brion) and Benmont are my biggest influences in the area of songwriting and general musicianship. It's really great getting to sit around and pick (Brion's) brain,î Watkins said.

Both Brion and Tench are featured on Watkins' new solo record, as is Wilco drummer Glen Kotche. Thile co-wrote the band's next single, "Jealous of the Moon," with Jayhawks leader Gary Louris. So, the band has had plenty of opportunities to absorb influences from several spectrums of music.

If you go by its Grammy nomination, "Why Should the Fire Die?" is a contemporary folk album. (Winners will be announced Feb. 8 at Staples Center in Los Angeles.) As mentioned, it's the same category that landed Nickel Creek its only Grammy, for "This Side."

At the time, Sean Watkins said Nickel Creek wasn't a folk band. There was no chamber-pop-folk-newgrass category. Still isn't, but if so, it'd be time to amend it.

"Eveline," highlighted by its harmonies, sounds like Nickel Creek interpreting "OK Computer"-era Radiohead. The band's third instrumental, "First and Last Waltz," isn't some fitful picker but a chilly effects-draped recital piece. Audible haze and forceful strokes of guitar introduce "Best of Luck," a percussive number about a doomed relationship that orbits around Sara Watkins' assertive vocals and features periodic gusts of atmosphere. Like it or not, the song sounds unlike anything the band has put out.

"I think (the CD) sounds like the first record more than the second one," Sean Watkins said.

Huh?

"It has more to do with the energy of the songs. I thought that the people who liked the first one would like this one better," Watkins said.

Certainly, song such as the title track and Thile's "Doubting Thomas" are matured bridges to the past.

"We feel more comfortable with who we are and our ties to bluegrass and country, and old-timey and Irish music. And on the second record ('This Side') I think we were consciously trying to avoid that."

The Nickel Creek on "Why Should the Fire Die?" sounds more, well, adult, even entertaining cynicism and sarcasm.

On "Can't Complain," the narrator answers his lover's attempt to date one of his friends by cheating on her with another friend. "But remember, I warned her and I'm a guy / So she can't complain."

The protagonist on "Helena" isn't much more sympathetic.

"Cause Helena, guys like me never sleep alone at night / I don't need your sympathy / I'll always be just fine."

Thile, who got married and divorced between "This Side" and "Fire," wrote both songs.

"He likes to explore that side of more characters that aren't him," Watkins said. "('Can't Complain') is from the point of view of a friend. 'Helena' is a character study about if you made all the wrong choices in a relationship situation that's what it would be."

Watkins penned a kiss-off for no one in particular, he said, called "Somebody More Like You."

"I had this clever line and decided to build a song around it."

That lyric is "I hope you meet someone your height, so you can see eye to eye with someone as small as you."

Much of the band's purity, musically and lyrically, that shone brightly on the first two albums has been chipped away, but Watkins noted that the band's former producer, Alison Krauss, played a key role in shaping the band's sound.

"Alison had a lot to do with how those records sounded."

The same can be said for the new producers, Eric Valentine and Tony Berg. "To put all the responsibility for change on us is not really fair.

"We just met him (Valentine) and it clicked and he got what kind of record we wanted to make," Watkins said. "The world's a big place. (Krauss) would probably suggest we work with somebody different. There's lots of different people to work with and we have huge ambitions. We want to do something different with every record."

Not everyone is going to keep following. Some listeners were probably grumbling when the five instrumentals on "Nickel Creek" shrunk to just one on "This Side."

"I'm definitely sure we've lost some fans. You can't make a record for the people who like the first record. It's definitely a coffee-table record," Watkins said, alluding largely to the folks who got swept up in the "O Brother Where Art Thou?" phenomenon. "Those people aren't necessarily going to buy the next two records. It' doesn't really matter to us."

"Nickel Creek" has sold almost 1 million copies, about 360,000 more than "This Side," according to a Sugar Hill rep. "Fire," which debuted at No. 17 on the pop charts and is the No. 1 bluegrass album in the country, still has a ways to go to catch "This Side."

Another noticeable change is the degree to which Thile's name dominates the credits, including five solo tracks to just one each by Sara Watkins and Sean Watkins.

"Chris writes a lot and sings a lot. The songs were chosen by the producer and you just worry about what makes the song best," Watkins said.

It's not uncommon to see or hear Thile referred to as the group's breakout star, that bright, extroverted mandolin seemingly more often than not the featured lead. Even on a key Sean Watkins tune, "This Side," it's a mandolin, not a guitar, weaving the melody. Watkins isn't so sure that perception is a correct one.

"Yeah. It really gets spread. Like the last record, we did three videos, two were songs I wrote and sang ('This Side' and 'Speak')," said Watkins, noting that the other song, "Smoothie Song," was an instrumental.

This time, the band's first single and video, "When in Rome," was penned by Thile. He co-wrote the next single, "Jealous of the Moon," with the Jayhawks' Gary Louris. (And Thile even sounds a little like Louris.)

"We have so much input in all of the songs. … What the public perceives, they can think whatever they want. We know that we're all extremely involved. We're all working and contributing. It's easy to be comfortable when you really are."