Hmmm...siguro, with regard to choral groups per se, you have to emphasize that choral groups in the Philippines enjoy the wide support of schools/universities and churches. Schools, colleges and universities, kasi understood yung rationale behind having a choral group, isang way ito para maengganyo ang mga mag-aaral na gumawa ng musika. And churches (both Catholic or non-Catholic), kasi they serve an important function in worship. IMO, church-based choirs are greater in number just by considering that there are more churches than schools in the country (but don't take my word for it, this is just a guesstimate, I don't have solid figures). Choirs that don't belong to these two groups are not many---for instance, there are just a handful of choirs that can be considered as "professional choirs" (e.g. wedding singing groups that normally don't have Sunday masses).
First, the Madz

Take note that
the Philippine Madrigal Singers is primarily a university-based choir, nagkataon lang na sila ang ni-designate ng CCP bilang resident choir and hence enjoys a status of being the unofficial "national choir" (w/c is technically correct, since the Madz, as a CCP Artist in Residence, enjoys government subsidy). The Madz have won awards, which have put the Philippines at the same level as other renowned international choir and which have encouraged other Pinoy choral groups to raise their artistic standards (and actually win awards themselves

) To further promote the cause of choral music, the Madz have various outreach programs and conducts annual workshops (usually, in October of each year) to improve choirs' skills. (Don't forget to mention Prof. Andrea Veneracion as the foundress, and Mark Anthony Carpio as her successor

)
The Madz, also, produced some of the country's finest choral composers, arrangers and conductors. This serves as an effective way of disseminating the knowledge that enabled the Madz to become a world-class choir: almost every major choir in the country is/was handled and/or trained by a Madz alumni. Each of these Madz alumni have themselves enjoyed successful careers as music professionals: notable names that shouldn't be missed out, among others, are Christopher Borela, Robert Delgado, Ed Nepomuceno, Anna Abeleda-Piquero, Arwin Tan, Jonathan Velasco, Eudenice Palaruan, Ruben Federizon, George Hernandez, Emmanuel Laureola and, of course, Ryan Cayabyab

)
You also have to mention, though in passing
lang, the San Miguel Master Chorale (headed by Madz alumni Ryan Cayabyab) as being one of the very few professional choirs in the country (i.e. it is the members' sole means of livelihood). Their recordings enjoyed popular and commercial success, which is rare for any choir besides the Madz's recordings. The group was disbanded a few years ago, however, and no official reason was given for the disbandment other than Danding Cojuangco's explanation that San Miguel Corp. thought it was better to invest their money in its basketball teams
If you can get any other sources for Pinoy choral music history before the Madz, so much the better, para mas complete yung description mo. I think that the book about Prof. Veneracion ("Andrea Veneracion: A Life Shaped in Music" by Anvil Books) has a few chapters with the necessary background history; if your library has it, I
ask request demand that you read it right away. Now na
As for choral literature---important na kasama ito, kasi kung walang magpo-produce ng mga Filipino choral pieces or arrangements, kulang yung identity ng pagiging Filipino choir

Parang kulang kasi kung may Pinoy choir nga, pero wala namang kinakantang Pinoy songs, right?

Marcelo Adonay, one of the earliest (if not the first) true-Brown Pinoy composers, wrote many sacred choral pieces such as masses and motets. Other Pinoy composers and arrangers contributed to the vast choral literature of the Philippines: besides the names I mentioned earlier (they have also composed/arranged their own pieces), we also have Fabian Obispo, Lucio San Pedro, Francisco Feliciano and Fidel Calalang (among others). Young composers and arrangers such as Nilo Alcala, John Pamintuan, Lester Delgado and the current crop of Madz arrangers (besides Chris Borela) are also contributing to Filipino choral literature. In addition, some religious groups are also into music publishing---pinakakilala na yata, hands down, ang Jesuit Music Ministry more than any other congregation-based or parish-/(arch)diocese-based group/entity---and this also ensures that the Filipino voice will become heard and published in the realm of sacred music. These composers and music publishers have their own groups who interpret (and record) these new compositions, also ensuring that more and more fresh works will be disseminated across the country and even outside. (And of course, this means, more songs for choirs to sing, and more incentive for people to form more choirs.)
Ayun...my (almost) comprehensive survey of Pinoy Choirs

Let me know if I missed something. For the rest of you, feel free to add anything that I may have missed.