There is no question that Eric Holtan and his four-year-old professional choir, Tucson Chamber Artists, are ready to take on Mozart's "Requiem."

The question is: What next?

To that, Holtan chuckles. First, he says, let's get through season No. 4. The season has, so far, been a true showcase for how far the choir has gone and the endless possibilities of where Holtan would like to take it.

"I think our recent programming including the Mozart C-minor Mass and works like Bach's Magnificat have positioned us very well to move to the next level of repertoire, and that being the Mozart 'Requiem' for sure," he said.

"The singers and the players, they're top-notch. They're Tucson's best. Our chamber orchestra is led by (Tucson Symphony Orchestra concertmaster) Steven Moeckel. They've played this repertoire. Even ambitious church choirs have done this work. But we like to think we're going to do it better. We're going to bring a level of artistry that's not been heard in Tucson before."

One listen to this ensemble and you realize it is not a baseless boast.

Holtan and the Chamber Artists — many of whom moonlight with the TSO Choir and the Arizona Opera Chorus — have quickly established themselves as a brave group, willing to jump in deeper with every concert.

Some would argue that doing Mozart's "Requiem" is jumping into deep classical waters.

It is a powerful piece, gigantic in scope and breadth from what you hear to how it came to be. Mozart died before the piece, which Austrian Count von Walsegg commissioned in July 1791 to memorialize his deceased wife, was completed. What was finished at the time of his death on Dec. 5, 1791, was played at his own funeral.

A debate continues in some circles as to who finished "Requiem," although history has pretty much agreed that Franz Xaver Süssmayr, Mozart's student, finished the bulk of it about 100 days after the composer died.

Holtan has paired "Requiem" with Schubert's Symphony No. 8 "Unfinished," which the TCA Orchestra will perform to open the concert.

"In combining the Schubert 'Unfinished' symphony, I felt that it brings an interesting character to the program," Holtan said. "These two works are held up as biggies in the rep, and yet they have interesting stories because they weren't completed by their composers by their death."

Schubert completed only two movements of his No. 8 and sketched out the third. Some historians and musicologists speculate Schubert simply lost interest midstream. There's also a theory that he became so consumed with other works that he put it aside and forgot about it. Then there's the school of thought that the composer believed the first two movements were so perfect they didn't need accompaniment.

Whatever the story, "Unfinished" has gone down as one of Schubert's finest works.

"There's no doubt this has the potential to be our biggest and best performance yet," Holtan speculated, then added, "I would think, given the repertoire, this will be a highlight of the concert season in Tucson."

The ensemble will be 28 voices strong for this weekend's performances. The orchestra numbers 34 musicians.

The TCA in April will perform another "Requiem," this one by Maurice Duruflé. That concert, which closes the 2007-08 season, also will include the world premiere of a TCA commission by Paul Crabtree, who sets texts from Ovid's "Metamorphoses."

Holtan said he already is penciling in dates and repertoire for next season.

Tucson Chamber Artists

• When and Where: 7:30 p.m. Saturday at Grace St. Paul's Episcopal Church, 2331 E. Adams St.; 2 p.m. Sunday at St. Andrew's Presbyterian Church, 7575 Paseo del Norte.

• Tickets: $25-$35.

• Program:

Schubert's Symphony No. 8 "Unfinished."

Mozart's "Requiem."