Was it imbalanced before the mods (a lot of people forget to check this).
The usual culprit for lack of bass would be a loose seal somewhere. Double check (particularly the right side) the pad sealing around the baffle, check for any tears. Try electrical tape around the pad edge to ensure a seal. If it's not that, maybe you accidentally blocked the vents in the right cup?
Edited by Armaegis - 2/19/14 at 3:37pm