Sealed or ported sub for an apartment?

Sealed. Apartment rooms are small, and small rooms add room gain below ~40Hz that compensates for a sealed sub’s gentle roll-off — you get the extension without the ported box’s output peak, which is exactly the energy that travels through shared walls. Add the practical wins (smaller box, easier level control) and sealed is our default apartment call. Then set the level a few dB lower than you want, and your neighbors will decide how long you keep the hobby.