First off, thanks to @philosophicalracoon @philRacoindie @philR_aco_, a few levels now have ACTUAL NES remixes and not just me slapping their MIDI's through a soundfont: Super Freaks Stadium, Skyway Highway, Toy Block Bridge (this one in particular goes hard), and Barrel Canyon Cliffs.
Second, there's been quite a lot of subtle balancing changes. Enemies spawn in a fixed shuffled pattern as opposed to being randomly chosen, which means guaranteed less of the dangerous enemies. The enemy portal spawn them much faster, but also shoot out yorbs if the screen has too many enemies for it to spawn more (still working on this particular thing). The end result feels a lot more consistently chaotic than before, but also a lot better balanced. You're not either screwed by way too many Skizzors/Pinheads or waiting around for anything to spawn in, and there's more incentive to leave the screen busy. I personally like it a lot better than before.
1