Yeah, the OST is awesome music for sure. If you have the chance, try to get the orchestrated (remade) OST in favor of the original one. It definitely adds A LOT.

As you guessed correctly, these "extra layers" are a number of extra pipe organ recordings that I played along and recorded live; after recording, I dubbed these onto the original ("the pure-orchestra part") and mastered the organ volume to achieve a balanced overall mix.

Thanks for listening and your comment, @pusen!