Toufu, your Quad 12L2s are rear ported right? These speakers are not really high output so maybe the rear port is vibrating the wall. It would not hurt to try the opposite wall but it will depend on the size of the room vs the standing wave. Do you have a subwoofer also?
Older apartments have paper thin walls, not sure if you are in modern condo with concrete-likely not.
I agree with talking with your neighbor to establish an agreement on level and hours. like in the city, eh? sometimes moving to a nicer apt means fussier neighbors where the other buildings tolerate all sorts of noise. Depends on the neighborhood. Are you in SOMA or downtown?
Let me know about the DAC demo too when this gets settled.
cheers, ed