You probably want to try to relocate where the sub will go. I think at the very least it's not going to be great to have the vent blocked. At worst, I would think there may be some long term damage to the sub that may occur. MDF is relatively dimensionally stable, as most composites are, but it does have wood product in it which isn't. The finish, whether it's vinyl or veneer or stain or whatever won't benefit from hot air blowing on it part of the year and cold air another. Over time I think you will find that you will have problems with the sub (best case cosmetic only, worst case would be glue line failures, etc.)