Sealing ported enclosure to reduce boom

"Sealing" the port is a bad idea. The drivers and crossover were designed to work with the port open, at least to some degree. The suggestions to play with positioning away from boundaries and partially blocking the port with foam or even a sock are legit.

Depends on the speaker, but if you have a three or more way speaker and multiple ported chambers, you could have some frequencies drop out if the mid woofer driver/crossover combo is "looking" to the port for bass reinforcement. Probably not common, but possible.