Depending on where the hole is, it might be possible to just remove the pad and look for a noticeable hole on the front plate around the speaker so you hopefully don't have to open it up again

In my opinion, rolled off sub bass tends to result from too much venting, from the front of the headphones, like where your pads are not the back of the cup. Its possible when removing plastic you opened up a pathway for venting from the back of the cup, to reach the front of the speaker/ where...

From what I understand it definitely shouldn't be the wiring. If the wiring was messed up, it should result in a quieter side due to the resistance, not less bass. and obviously a short circuit wouldn't work at all.