thank you! the sub-bass boom in the latter half is actually a recorded upright piano (specifically a chord, not me smashing the piano). although not my original intention when recording, i thought the sound had a really interesting texture and decided to run it through a sub-bass plugin for a unique 'boom' :D
this kind of sound is used all over the soundtrack of Hyper Light Drifter (composed for by Disasterpeace, of whose work i have drawn great inspiration from) and i think it's really effective at creating the perception of a massive space!