Brothers Isaac, Taylor and Zac Hanson [ tickets ] trade in their bubble-gum pop for a more mature sound, which the trio showcases during its current acoustic outing.

The run, which kicked off in early August, now stretches into mid-September, and features stops at clubs throughout the U.S.

Hanson's tour is designed to give fans a look and listen to a group that has changed significantly since it burst onto the music scene with the 1997 teen-pop hit "MMMBop"; back then, Isaac, Taylor and Zac Hanson were 16, 13 and 11 years old, respectively.

The brothers are previewing material from their next studio set, "Underneath," which they had hoped to release this fall, but now expect to release early next year, according to their website. The band has been without a label since splitting with Island/Def Jam earlier this year over creative differences.

Hanson's website describes "Underneath" as a "diverse collection of self-produced and co-produced songs [that] are both lyrically stimulating and filled with classic musical hooks but the more mature sound is not without the trademark Hanson harmonies and classic soulful vocals."

When it does surface, "Underneath" will mark the group's first studio set since 2000's "This Time Around." Musicians who collaborated with the Hanson brothers on the new album include Matthew Sweet, Carole King, Greg Alexander (New Radicals), Ed Robertson (Barenaked Ladies) and Michelle Branch.

While the full-length, plugged-in version of "Underneath" won't surface for some time, fans can get an early earful of some of the new material on the just-released EP "Underneath Acoustic," which the group is selling at its website, as well as at its shows.

Hanson recorded "Underneath Acoustic" at the Church Studio in Tulsa, OK, in early July. Members of the group's official fan club attended the private session.