If I might offer an alternative to blocking the ports... pull the speakers out into the room to minimize the room reinforcement. Blocking the ports reduces bass output from the speaker to compensate for bass reinforcement by the room... the other alternative is to leave the speaker "flat" and remove or reduce the bass reinforcement from the room.

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8