When it comes to new music in this city, perhaps the Pittsburgh New Music Ensemble springs to mind first. It has "Pittsburgh" and "new music" in the title, after all.

But don't forget Cuarteto Latinoamericano, another musical pot-stirrer. The string quartet, in residence at both Carnegie Mellon University and the National Fine Arts Institute in Mexico City, has been commissioning, performing and recording new music -- especially by Latin American composers -- for 25 years.

Tomorrow night at Carnegie Music Hall, the Cuarteto will join the Spanish guitar quartet EntreQuatre for an unusual event -- performances of three pieces of new music for double quartet, as well as music played by the two quartets separately. A string quartet by the 20th-century Brazilian composer Villa Lobos will be about as close to standard repertoire as you'll hear.

Saul Bitran, the first violinist for Cuarteto Latinoamericano, makes the case that small ensembles such as Cuarteto and EntreQuatre are the most practical and artistically satisfying vehicles for new music.

"There are a lot of orchestras that are not in good situations now, some of them collapsed, and there are more string quartets than there were before," he says. "It shows that presenters and music organizations realize that string quartets have the best repertoires. ... It's much less expensive, it's easier to move around. We only need four chairs, and we even travel with our own music stands."

Certainly, the Cuarteto has thrived in the past 25 years. But their story begins even before that. The quartet comprises three brothers -- violinists Saul and Aron and cellist Alvaro -- as well as violist Javier Montiel.

Saul was born in Mexico, but his two older brothers were born in Chile, where their father was an economist and amateur violinist. His father had worked in the socialist government of Salvador Allende for a year while on leave from his regular job with the United Nations. When Pinochet's brutal coup toppled Allende in 1973, the elder Bitran left quickly.

"When the coup happened, he was already back with the U.N., but he didn't want to risk it, so he took the first commercial plane out of Santiago," he says. "We stayed there and we sold the house and then we joined him in Mexico City a couple of months later. Many of his colleagues were arrested or worse than that."

Eventually, the Bitrans were able to move back to Chile. The family also spent two years in Brazil, when their father was posted to Rio de Janeiro.

That Brazilian sojourn led to the Cuarteto's recording the entire Villa Lobos output of 17 string quartets, earning them nominations for both a Latin and a chamber music Grammy.

The members of Cuarteto Latinoamericano lived in Pittsburgh year-round from 1988 to 1994. Now they do visiting residencies at CMU four times a year, with a similar arrangement in Mexico City.

Here's Bitran's brief tale of two cities: "They couldn't be more different, and that's what we like. Those contrasting experiences. Mexico City is chaotic, it's full of life, it's noisy and polluted and traffic is unbearable. But there is a lot of culture and vitality, a thriving musical scene, the museums are great.

"Pittsburgh has great museums and great musical life, but it's quiet, easy and manageable. The weather in Mexico is pleasant year 'round. In Pittsburgh -- you know how it is."

The Cuarteto also spends about six months a year touring.

In Europe in 2000, they met EntreQuatre, a veteran guitar quartet based in the mountainous province of Asturias in Northern Spain. The groups have premiered new works for double quartet and recorded a CD, "Tierras Juntas."

Pittsburgh has seen at least two well-established guitar quartets in recent years -- the venerable Romeros and their edgier proteges, the Los Angeles Guitar Quartet. Unlike those groups, EntreQuatre remains relatively unknown in the States. But Monday night's audience can expect some fireworks.

"This group EntreQuatre is particularly strong in rhythm, they play very, very precise and with a lot of fire," says Bitran. "So that was a great thing for us to find, because we can really do some very precise rhythmic attacks, and everything sounds very powerful."

The program ranges from Alejandro Cardona's "Cabalgando Vientos," with its eerie sounds of sighing winds, to the more lyrical and tonal "Concierto de Terra Alta" by Jose Luis Barroso. Bitran places Barroso's piece "in the tradition of Joaquin Rodrigo," who wrote the most popular piece ever for guitar and orchestra -- the Concierto de Aranjuez.

Bitran attributes the wealth of new music for chamber ensembles to composers' "realizing that rehearsal time with orchestras is limited and very expensive, and they're mostly not satisfied with the performances they get," he says. "They sit with us for hours and hours. We feel that composers are achieving much more of their personal message through these string quartets. ... No matter how good an orchestra is, they will never get the level of subtlety that a quartet can offer."

Cuarteto Latinoamericano doesn't always have to look so far afield to find new music performers and composers. They have worked with Pittsburgh composers from Reza Vali to Nancy Galbraith to Leonardo Balada.

"As we speak, we are learning David Stock's Quartet No. 7, which we will premiere in Pittsburgh in April, and we are recording it in Mexico City on the 13th of March. We love to explore the Pittsburgh composers."

Like the rest of the classical music world, Cuarteto is keen to pull in younger audiences. Bitran credits the Kronos Quartet, with its hip look and genre-bending repertoire, for helping in that regard.

"It's a major component of our career. If we go to a place and play a concert, we always try to stay the morning after and do a residency at a local school."

It would help, Bitran says, if classical radio would play more chamber music.

"We still think that the role of classical radio is more important than anything else in exposing people to classical music. And we do feel regret. Most radio stations in the country have decided that chamber music is for experts, and for some reason they can only broadcast it late at night. They have this feeling that a Tchaikovsky overture is somehow easier to hear than a Mozart string quartet. I'm not sure that's an informed decision."

Then there are the two Grammy nominations, which you would think would have raised the profile of Quarteto Latinoamericano.

"We were expecting it, but it hasn't happened and I don't think it will. I'm not complaining. We have a good career, but I think we have the same career we had before that."